I was going to buy some wheels next month but I can't decide on 19's or 20's the of the rims will be

front 20x8.5 rear 20x10 with a 2" drop

 

is there a reason to go 10" wide at the rear...?
you can still lower it depending on the offset...

 

There's gonna be very little room to drop it.

Your stopping and handling will suffer.

You wlll not be able to hit 0 to 60 in 8 sec.

 

this is what i've experienced, hope it helps...i got 20''s. only cos i could'nt get the style i wanted in 19''. my suggestion is to get 19''s, unless u gonna turbo ur 8. cos the 20''s slow the 8 down alot and when i had my 1st set of wide 275/20''s, i found the 8 feeling very much like if example the RH wheel went over a small dip in the road it would pull to the RHS. it was'nt pleasant, now have 245/20'' on and feels better. especially with the drop ... i've managed to lower mine, S.A measurements in mm. 15mm at the rears and 30mm in the front. had a arch trim at the back cos it was scraping. now no scraping and wheels fill the arches alot better than the 18''s.