Is it possible to run a set of 20" x 8.5 F (offset +20) and 20" x 10 R (offset +25)? also what size of tire should I go with. Im dropped on H&R springs. Camber kit only in the rear.

You can probably snug them in, but you'll have to go the stretched, cambered route more than likely.
Here's a monster thread on aggressive, flush fit Gs, mostly coupes but I believe there's a decent amount of sedans in there.
http://www.myg37.com/forums/wheels-a...sh-thread.html

id go +35 on rear. you can fit 275/30/20 with no rub or stretch.

I've got 20x10 +45 in the rear and I'm running 295/30/20 without issue.

I do want a mild stretch not too aggressive. I was thinking 255/35/20 maybe on the rears

sounds a lil too close for comfort IMO