Octo for mk17?

[Mobulai]

My 4 cents (this is a bit long)

This I would answer in 2 pieces:
It does matter, within rec warm water limits OOA/G situations sneak up on you, besides being stressful mentally, they can be also physically
30-40m on Air is still “rec”; and gas density there is quite high, and close to comfortable limits.
If you are hyperventilating because of fighting current or panic due to OOA, that get you to a “risk” zone, not full risk but let’s call it “yellow light”: CO2 offloading

Just supplementing my train of thought about WoB and CO2 loading

This is absolutely correct.
You will be able to breathe a tank down further with little no increase in breathing effort using an unbalanced 1st stage like a MK2/G250-G250 as opposed to using something like a MK25/R190.
People think only one stage needs to be balanced, which is true up to a point. The point is when the tank pressure begins to go below the operating IP of the 1st stage, then the unbalanced R190 will begin to breathe hard because the spring pressure of the R190 is set to whatever normal operating IP is.
But with a high quality pneumatically balanced second stage, they can compensate for pressures much lower than normal IP, in some cases down to 10 PSI and in shallow water breathe just fine essentially on fumes. So a MK2 with a G260 will be able to be breathed further down than the MK25 with an R190. I don’t know if this is good or bad, it is a fact though. Personally I’d like a little warning if I wasn’t so much of a gauge checker, which I am.
All this regulator hair splitting should be moot because any diver should know to be well on the surface before any of this low tank pressure and hard breathing becomes an issue.
So just get whatever you want based on your budget and personal likes because in the real world it really doesn’t matter.

 

[buddhasummer]

The plastic orifices are fine. I find that I cannot set them to the finest edge like I can a good brass or steel or gold orifice but they hold tune much longer because they do not cut the seat. All in all, I am going to say they are an improvement. I have had no problems with durability.

I am going out on a limb a little bit here but I might say the plastic orifice negates the need for a seat saver function.

Agree wholeheartedly, admittedly initially price was the driver behind my switching to plastic but after having now used them in a fair number of regs I've no complaints.

Unlike James I've noticed no discernible difference in tuning capacity between the two.

 

[buddhasummer]

However, where practicable/possible my preference is metal over plastic in most things.

 

[Nemrod]

Agree wholeheartedly, admittedly initially price was the driver behind my switching to plastic but after having now used them in a fair number of regs I've no complaints.

Unlike James I've noticed no discernible difference in tuning capacity between the two.

It is not a huge difference, just noticeable when fine tuning. I like to set my personal regs up such that they barely have a tiny leak with the knob fully out which is NOT IAW the service documents. I find the plastic orifice either wants to seal or it leaks too much and I have to fiddle with it on the G260 micro-adjust to get to just a few bubbles. I usually do not do this final "fine" adjustment until the regulator has a dive on it or at least a goodly number of cycles so that the orifice and seat of whatever nature they are have become acquainted with one another a little bit, hopefully in a favorable way .