One of the problems I have seen with what little research I have seen and what most people experience is that I don't believe the issue was pushed enough to make a noticeable difference. If I recall one study correctly, they had divers perform two pretty benign dives and report how they felt. I don't think anyone can tell a difference under those circumstances. If the tiredness factor is actually caused by subclinical DCS, then you have to push things to those limits to see if really makes a difference.
My very first liveaboard experience was only three days of diving long, off the Great Barrier Reef. I averaged 4 dives per day, all on air. The depths were not great--I don't think I ever hit 100 feet, and I was diving AL 80s. I was in excellent physical shape. By the third day I was totally whipped. I couldn't even eat dessert, and if you know me, that is amazing. I went right to bed after dinner and had to be awakened the next day, after sleeping about 11 hours.
I have since done several more liveaboards, all on nitrox. On one of them, most of the dives were 100 feet or deeper, using steel 112s. On each of those trips, I felt great every day and was up having fun with everyone else until it was clearly time for bed. I never once felt what I did on that first trip after diving air for only three days.
Now, that is anecdotal evidence and by itself means nothing. I believe the way to tell if nitrox makes a difference, though, is to do some sort of test like that. You have to stress the divers enough to make any difference readily apparent. If what happened to me is actually typical you won't need any kind of subjective evaluation. Researchers will see the difference easily.