Are there ANY double blind studies?
I couldn't find any. I found one that showed a small improvement on cognitive performance during the dive, but not clear why.
DAN published something about this issue in 2014, asking RIchard Harris and Neal Pollack about this issue, and they agreed that there wasn't literature support for it. Not sure if there is more recent data. Paging @Dr Simon Mitchell ...
If people say they "feel" better and there is no scientific evidence to refute the potential for feeling better - then I would be inclined to believe people.
Well of course. If you feel better, you feel better. It's not really a scientific question, although it can be studied scientifically with a double blinded study.
Just be careful about terms. The burden of proof in science is on the positive. And you don't "refute the potential", unless you have identified an underlying mechanism (which hasn't been done). You can test the hypothesis, though. If randomized blinded divers claim that they feel better just as often diving air as they do nitrox, then that suggests that it's all placebo effect.
I understand that if one simply looks at an equivalent amount of super saturation of nitrogen a person is subject to (and the ascent rate) they use, then I can see why someone might suspect the placebo effect.
On the other hand, could it not be possible that a safety stop on nitrox is somewhat different in that there really is a higher partial pressure difference when using nitrox and this makes it "better" in some small way? It's more of a question than a statement.
Yes, those are interesting questions. What you really need to do is to find out of nitrox divers have less decompression stress than air divers, and if so, why? Are they using different algorithms or different conservatism factors? Do they have individual differences in their ability to offgas nitrogen? Is it a self selection process?
I'm fine with people doing whatever they want to feel better, but when advising new divers, it's important that we make it clear that just switching to nitrox and then having your dives limited by NDLs may not really give you any improvement in either DCS risk OR post-dive fatigue.