power ratings are just whatever team is 'hottest' at the moment. it's like you take out divisions and conferences, and have the standings that way. but it figures in for how many games have been played, stuff like that. i know i didn't do very well explaining that- but have a look here:
http://sports.espn.go.com/nba/powerranking
THAT'S what power rankings are.
As far as turning them off in dynasty, I don't think so. At least, nobody has figured out how yet. I agree that it's annoying.
I'm pretty sure that doesn't effect your GM rating. I don't see why it would- because alot of the things that give you your GM rating also give you your power ranking. So that would be overkill if you team's power ranking affected your GM rating.
Hope this was helpful (and not too confusing- it is pretty late

)