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.