My own belief is that before we start trying to get rid of nitrogen with helium or hydrogen, we ought to check first that increasing oxygen doesn't deliver some or all the benefits.
Well, you can't increase it too far. The fire hazard gets insane pretty quick. 30% O2 is probably OK, but does have a substantial fire risk increase; 40% probably isn't. Also, increased oxygen has long-term health impacts (I don't remember details; I could look them up if you're curious), but I don't think we know what level those start at to any precision.
I suppose fire risk isn't a huge deal if you're using portable breathing tanks, but you do still need to investigate health impacts. They're long-term enough that you could ignore them for initial study of the cognitive effects, though.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.