The Scubapro "High Performance" second stage is numbered as the 080, but seems to be frequently referred to as the "108". I don't know where that comes from other than the next second stage SP introduced was the "109". Of course, the name "High Performance" never made much sense either as it was SP's basic first stage and did not have as high of performance as the 109.
In any event, the 080 is still capable of very good inhalation performance as it is a very smooth breathing regulator (compared to the crappy plastic entry level second stages made today) and is easy to adjust to a very fine level of inhalation performance and that level of performance is useable in the water due to reasonably good case geometry fault issues. It is not uncommon for one to function reliably with no tendnecy to freeflow at cracking efforts as low as 1" of water and it has a moderate venturi effect that requires little or no sustained inhalation effort without forcing air into you.
It's weakness is that it is not very tolerant of intermediate pressure changes. For exmaple if you tune one being used on an unbalanced first stage for the lowest possible inhalation performance on a low pressure (2250 to 2400) psi steel tank and then place it on a full 3000 psi AL 80, it will freeflow due to the higher tank pressure and higher resulting IP.
Similarly, if tuned for minmum inhalation effort at 3000 psi, it will breath significantly harder at 500 psi due to the lower IP that results with that lower supply pressure feeding an unbalanced piston first stage.
When used as an octo on a balanced first stage with very stable IP, the 080 will provide low inhalation effort regardless of tank pressure.
The R156 on the other hand, all things being equal, should be capable of the same 1.0" of water inhalation effort and will do it across a much wider IP range with very little degradation in inhalation effort. So I suspect that not all things are equal.
The problem may be a worn or damaged orifice that cannot be tuned to the same low cracking effort. Or it may be an issue with more mechanical resistance due to a corroded lever or an older lever that is not fully compatible with the new S-wing G250/S600 poppet. If the tabs on the lever are too large they drag on the poppet and cracking effort is increased.
It may just not be properly adjusted. Ideally, the 156 should be adjusted so that it is either just short of or has a slight freeflow when the adjustment knob is all the way out. If however, the tech adjusts the reg for an inhalation effort in the 1.2" to 1.4" of water range with the knob out, it cannot be diver adjusted for the lower performance that it is capable of acheiving.
