Oh that's really funny! I will qualify and say that I think I got up to 4 brain power, and I noticed the difference around 3. Once I got there, I could feel the difference, but I figured that out on the second or third playthrough, as I died rather quickly while still getting a feel for the game and wanting to see what the difference in weapons was. So initially, I wasn't sure what the brain power effected until I found my groove in the game, and then it was more apparent. So maybe the difference across players lies in that, and the effect of the brain power needs more noticeable gradient in effectiveness.
Obstacles would be a great level design feature! Hadn't thought of that.