I have seen both sides of the issue: I went with 20" wheels on my 05 Mustang GT (Black) and was not happy with the negative performance aspects of such a large wheel. The ride was compromised with the thin side wall and I had a horrible time with vibration when driving over 50mph, the steering wheel was all over the place. Had them road balanced twice and it only help the vibration a little. They looked great, but not worth it in my opinion. I traded the 05 for my 07 GT (red) and went with 18" wheels and love the look and performance/comfort. 18" is the biggest I would go since performance is my first priority. I plan on putting a performance set of coils that should lower the car about 1" which will tuck the wheels up nice and should help with cornering.
Jim - Beaverton, OR
2001 Ford Mustang GT
Patriot Performance Stage II Heads, Comp Cams Stage II Cam, MAC Ceramic Long Tube Headers, MAC Pro Chamber, Magnaflow Dual Exhaust Catback, Accufab 75mm Throttle Body, Steeda Full Length Subframe Connectors, Eibach Pro Springs -2", 3.73 Gears, Steeda Tri-Ax Short Shifter, K&N Mustang FIPK Air Intake,SCT X3 Tuner, CDC Shaker Hood Scoop 01 Cobra Seats, 326 RWHP