With a difference in tire O.D., you'll need a tuner to adjust the tire revs per mile so the speedo is on but it'll also change your overall drive ratio. It's probably better to stick with or as close to the OE tire O.D. Other than the minor change in rotational mass with the larger wheels, you shouldn't notice any performance change unless you DO change the drive ratio with a different tire O.D.
07 GT Prem: Alloy, MT, 3.55's, Steeda CAI w/Brenspeed tunes, FRPP Stingers, 275/40/18 Goodyear Eagle F1 A/S on SVT Cobras (OE GT500's), 14" shorty, Wheelskin side scoops & qtr window louvers, A-H rear window louvers, SHR honeycomb blackout panel, Sequentials, Hella Optilux Extreme XY fogs, Silverstars
06 V6: Legend Lime, Pony Package, 14" shorty, Sequentials, Hella Optilux Extreme XY fogs, Silverstars