I dunno. I'm currently focusing on very low rep (trying to improve my 1RM) stuff at the moment. I know what I can and can't lift, if I can make one more rep or not, no amount of "focus" (still not sure what you mean here) is going to grant me any additional strength. I don't disbelieve that the stuff can make you feel better, feel more energized, but when you get right down to actually lifting more, I can't see it actually helping me. tbh its usually when I feel like shit and expect a horrible workout that I surprise myself and do my best.
I also really dislike something that over time will make me feel like shit if I don't take it.