Yeah I definitely agree with the p90 is way OP...in fact, every game ive played that has the p90 in it, they make it OP!
I read a really good article recently about guns in videogames. They spoke to the gun guru at Ubisoft Montreal and he said that they first make all the guns for realism (shotty's really don't blast through armor, uzi's accurate, etc) and then they adjust for gamer mindsets (shotty's rip through everything, uzi's have poor accuracy, etc). Sorry I don't have the link, but you can search Kotaku for it...really interesting read!