Go with an 18x9 rim FIRST and formost! then get ya some Eibach pro springs and settle that body right down over those wheels and then you got a stang with awsome handeling and a killer stance! I am running that same set up and am perfectly happy with everything.
With those 20s your seriously scerwing with ALL performance of your car. The wheels are so big it takes MORE power to turn em over (slower car plus more gas too) then they weigh so much it takes too much to stop them things from rolling. AND the main thing peeps never think of when doing big wheels is... In emergency manuvering, or loss of controll (think hydroplane, or a schmoe pulls out in front of you) ... Your car as it is sits SO high you are MORE likely to roll over just like an SUV. Even if you never have an emergency come up.... who wants a mustang that doesnt perform? I would be scared as hell to drive a new edge stang on 20s.