I tend to favor the possibility that some augmentations may be incompatible with each other, or are likely to run into conflicts. Considering that this game is supposed to be a hard scifi game (or at least contain strong scifi elements), I think the theme mixes with this game quite well. For example, right now the current world record for a person holding their breath under water (doing nothing) is over 20 minutes. In studies done on such people, they've found that they have a much larger red blood cell count than normal people. It seems that their training has caused their bodies to grow much more red blood cells. They believe their bodies have done this in response to their training much like how the body will increase muscle mass in response to physical exercise. I think that the holding your breath ability of the "enhanced respiration" bioware augmentation works in a similar way. The augmentation provides more and better red blood cells. However, I think that the "respirocytes" nanoware augmentation (which also provides a hold your breath ability) also occupies space in your blood. At some point, I think that there is point where the only way to improve the ability to hold your breath by stuffing stuff in your blood, is to take something else out. In this case, respirocytes is the better augmentation to hold your breath, so that is the one that applies. I don't think that they should stack. You certainly can get the other benefits of the enhanced respiration augmentation since the other aspects don't compete for room in your blood. The "oxygen reserve" cyberware augmentation is a different augmentation that provides oxygen through a different means (it provides oxygen to the lungs instead of occupying volume in the blood). As such, I think that it stack with respirocytes and enhanced respiration.