The Mk 19 is indeed a Mk 17 with a swivel turret for the LP ports - just like the Mk 18 was a turreted version of the Mk 16.
It would be a good choice if you like the swivel port turret and want that combined with the fully sealed Mk 17 design.
The "overbalanced" thing basically makes me want to scream every time I hear it. It means absolutely nothing in regard to how Apeks uses the term.
The IP in any regulator maintains itself at a constant pressure over ambient pressure. The theory with the Apeks "over balancing" is that the relative difference of the IP over ambient pressure will increase as depth increases so that, for example, if the IP were 140 psi over ambient at the surface (155 psi absolute), it would be slightly more than 140 psi greater than the 65 psi ambient pressure at 132 ft. So rather than 205 psi absolute, you might have 210 or 215 psi.
The purported advantage of this is that the higher pressure will increase flow rate and offset the loss in flow caused by the higher viscosity of the denser air at depth. The problem with this thinking is that viscosity increases are not even a remote factor at anything even close to sane diving depths. At 600 feet or deeper - it might start to become an issue.
The second problem is that if the effect did occur, it would just screw up the tuning on the second stage as the reg would need to be detuned to not freeflow at the higher intermediate pressure found at depth. Alternatively, you could reduce or remove the downstream bias from the second stage poppet/shuttle valve. That however reduces the ability of the second stage to vent excess pressure in the event of a leaking high pressure seat and is unsafe.
More importantly, for the "overbalancing" to have any noteable effect on flow rate, the flow rate would have to be incredibly marginal anyway and it would in short not be a reg you wanted to dive with if it needed overbalancing to get improved performance. So the term overbalancing as Apeks uses it is total and complete marketing BS.
Scubapro defines "overbalancing" differently so the two are apples and oranges terms.
To set the stage, a "balanced" first stage essentially means that the intermediate pressure will not change due to the effects of changes in the downstream force of the air coming from the tank.
In an unbalanced piston reg, this change in tank pressure would cause a progressive decrease in IP as tank pressure fell from 3300 to 300 psi. In the Mk 2 for example a 20 psi decrease in IP is normal over that pressure range. In a diaphragm reg, the opposite occurs as the seat carrier is upstream of the orifice, so the IP in an unbalanced diaphragm first stage would increase as tank pressure falls from 3300 to 300 psi. This change in IP would affect the tuning of an unbalanced second stage resulting in slightly harder breathing at the period of the dive with the lowest IP. With a balanced second stage, the effect would be less pronounced but would still be present as a balanced second stage still has some downstream bias for the purpose of providing overpressure relief in the event of a leaking HP seat.
In a traditionally balanced piston design, the diameter of the piston stem was constant from the place where it passed through the high pressure o-ring sealing it against the reg body to the knife edge on the end of the stem that seals against the orifice. In effect this meant that the reg was not perfectly balanced as the area affected by the downstream force of air from the tank was very slightly smaller than the area of the piston stem due to the small but still present area of the sharp edge of the knife edge. In practical terms this meant the IP dropped about 4 to 5 psi over the 3300 to 300 psi range - not enough to be noticeable, especially with a balanced second stage, but still not perfectly balanced.
In the Mk 20 and Mk 25, the piston stem flares to a slighly larger diameter at the knife edge so that the area of the knife edge is accounted for and the piston is truly balanced with no I change due to tank supply pressure. SP calls this "overbalancing" as "overbalancing is apparently cool. It is also no doubt pandering to the market of ignorantly indoctrinated divers who think that Apeks version of overbalancing is important, but at least it means something.
The same is apparently true with the Mk 17/Mk 19 in that the area of the balance chamber is exactly the same as the area of the orifice creating perfect balancing - but that has never been an issue with the diaphragm design anyway, so it is again just marketing hype to compete with the Apeks marketing hype.
With regard to cold water performance, SP did extensive cold water testing with the Mk 17 at high flow rates at 165' in 35 degee water and the Mk 17 was the only first stage from any manufacturer (including Aqualung/Apeks) to survive the full 200 minutes of the test. It is as close to bullet proof in extremely cold water as you can get with a first stage and it is also an excellent performer.
