Generally speaking when psykers push themselves, esp human ones they start to bleed from their orifices ie, eyes, ears, etc. Some of them suffer haemorages and strokes and can kill themselves if they try to channel more warp power than they can control
In terms of raw power they are limited to their innate ability. Ie. alpha- class, beta class etc. Practically speaking there generally arent any normal human alpha class psykers as they shine so bright they are devoured by demons, or driven insane, or the imquisisiton kills them as being too dangerous to let loose if they turn.