Okay I'd forgotten about spear fishermen. Nice Video, I can see why you need to cover distance. somewhat barren and seemingly devoid of critters? 
Oouch
We can agree that the conventional BCD isn't the best for a number of reasons, however they are the most commonly used device.
What ever type of tank you use, generally you overall weighting is just enough to keep you neutral (+or - depending on your preference) at the end of a dive.
I guess I am just looking at this from a different perspective....I see the tank and BC is co-evolved gear.....and see it as typal degeneration of species Wetsuits and Drysuits just furthered the de-evolution. Dive Shops and DEMA just "sold" each year's new solutions as the way to go, and this was the "progress" that derailed a better evolution of dive gear. Remember how in the nineties, the new "Must Have" better gear, was all about BC's with bigger pockets, and brighter yellows or reds or other colors that jumped out in the show room--there was no thought what so ever, to actually improving dive efficiency and function....not until Halcyon around '97, but that's another rant
The old steel 72 did not require a bc at all....the larger tanks that followed, DID require a BC because of the weight they imparted.
In reality we don't use much gas in our BCD - in horizontal trim if you squeeze your wing its not that inflated and of course because you can compress gar as you go deeper and add more, your wing doesn't grow too much in profile. the majority of a BCD's buoyancy is required at the surface. So ignoring the parachute effect of a BCD a wing is pretty streamlined already.
As you are agreeing that jacket style BCD's are like parachutes....we are on the same page here....and I do agree that an 18 pound wing, or 30 pound lift wing are fairly streamlined....However....I think you might be shocked if you jumped in to tropical water with just a steel 72 and a backplate harness...no wing....there is actually much less drag....it figures in to something you feel more with kick and glide for frog kick...you glide much farther...and if running at spearfishing pace, or scootering, the comfortable top speeds you can easily sustain for an hour become much higher.
You could make it smaller of course simply only big enough to control buoyancy at depth. So at the surface you'd need something else, which is something else to go wrong etc etc. Not very DIR?
So back in the 60's and 70's....when people used 72's and j valves...and no stinking bc's....do you think most drowned at the surface? Of course you dont---- the surface proceedure was just different....divers popped on their snorkel, and could be comfortable on the surface for hours or days with no stinking bc
Today, you could also tow a Riffe or Omer Torpedo Float, so the boat could follow...this is essentially no drag, and it has handles on the side, with positive buoyancy of about 150 pounds. If you wanted, you can hang on to one like it is a raft--or you could have a student or 2 in your charge, grab on to it and keep their heads comfortably way up out of the water, for chatting on the surface..... In South Florida, we have to tow a float anyway for the boat to follow....we often drift and swim for many miles on a single dive.
A cylinder is already streamlined - the different diameters don't really affect the cross sectional area, you could improve the drag with a removable duck tail. Thebiggest streamlining would need to be at the first stage and hose end.
We would have to do a drag test behind a boat I think...but I feel like there is more drag in the wider low pressure 120's than the narrow but long hp120.... Length does not increase drag...it may reduce it as it does with boat hulls.??
If you covered the wing, perhaps tank and the wetsuit in denticles as sharks do then you could reduce drag by as much as 15% on moving fins by 6% (Harvard University in Cambridge, Massachusetts, study published (May 14) in the Journal of Experimental Biology.)
You would need to streamline the mask, get rid of D'rings and all the other stuff that disturbs flow.
Agree the mask is terrible...Freedivers could benefit enormously from a complete full head mask, that totally re-shapes the water flow....it should also extend to the shoulders, completely changing the leading edge as our bodies move through the water.
I agree the exposure suit should contain the harness.....and d rings would need to pop in and out of recessed areas, but easy to access with touch....All we need is one for the hip for pressure gauge..hose could be routed almost in the suit..in a suit channel that streamlines it when not being pulled out to read it....and we need this kind of hose routing for the long hose and necklace reg....they need to be recessed unless deployed in emergency scenario. Ultra slick outer fabrics exist....high tech non-buoyant insulating layers are easy to build into this.
Make the diver slick, and suddenly better fin technology makes a lot more sense..as does fitness ( it exagerates the differences)
[video=youtube_share;7ihkg4niGE8]http://youtu.be/7ihkg4niGE8[/video]
In theory possible of course but worth while only for hunters. Serious photographers and videographers have camera's so large in their housings and with lights they account from more drag than anything.
I dive with a canon 5 d mark II in an Aquatica Housing. With light arms and strobes, it has lots of drag....too much. Since I shoot mostly video, I dropped the arms, and have a powerful video light on top of my right wrist ( similar to goodman handle style, but better)....When I shoot, my right hand is at the top of the dome shade, pointing down, and I have mostly just the drag of the camera. It would be less drag without it.....sort of like telling a spearfisherman they should leave their gun on the boat
Without the drag of the arms and strobes, I can still outswim scooter divers for up to an hour, as long as we are not much deeper than 100 feet...too much deeper and Co2 levels become problematic at that power output....the point is though....you can be much slicker, and do much less work to move along in the water...and DEMA sponsored de-evolution of dive gear needs to be counted by consumer interest.
---------- Post added June 9th, 2015 at 07:26 AM ----------
More DE-evolution thanks to the "Dive Industry" and DEMA....
The de-evolution of the regulator.....
While they did initially improve the single hose regs from the double hose regs everyone saw on SeaHunt...this stalled a long time ago....
Fortunately, some smart divers began working on IMPROVING the regulator again.....and now we have the Kraken, a double hose regulator, which one day soon will most likely exhaust tiny "filtered" bubbles...much like "fiz", out the back exhaust, far away from the diver's vision, and silent....For the recreational diver now WASTING upwards of 20 grand on a nonsense rebreather and all the wasted training( wasted because they will still be many times more likely to die with this set up, and because there is a better OC solution for dirt cheap) , instead they can have a double hose regulator that is as silent as a CCR rebreather, allowing unprecedented approachability for many species of fish....for more like $800 and with no special training required....
There is no disputing the advantage of a rebreather for some cave and very deep diving applications....but these are dive "missions" where it is accepted that the risks are huge compared to recreational diving--so the vastly more dangerous CCR ( which is so highly profitable to the DIVE Industry) becomes less ridiculous from a functional standpoint.
Personally, I see DEMA almost as dangerous to divers as Monsanto is. It is profits far over function...it is how much can we make, not how many people will this kill.
