The car would probably handle and brake better with the bigger tires up front and the smaller tires in the rear. With 518 rwhp, it ain't going to matter which tires you have on the rear for grip purposes.
You are going to need to either lurk on Corner-Carvers and use their search function for a month OR buy a copy of Race Car Vehicle Dynamics and read it to learn about roll centers to get a good answer to this question. Playing around with Bill Shope's suspension website is also a good idea (google it).
Reader's digest version: It's not optimal. 245/45-17s are 25.7 inches tall. 255/55-17s are 28.0 inches tall. That's a difference of 2.3 inches, which means that the rear axles are 1.15 inches higher than the front spindles. On a tiny level, this actually shortens your effective wheelbase (think about what having 5' tall rear tires would do to the car. As the ass end lifts up, the horizontal distance from front contact patch to rear contact patch lessens AND you have more weight on the front wheels of the car). Is this a huge deal? No. Is it optimal for handling? Nope. Unlike Porsches, Vettes, and other cars with different stock tire sizes front/rear, stock Mustangs, imo, were designed to run the same wheel/tire size front and rear (although some newer special package Mustangs may not be like this anymore, I dunno).
If you raise the ass end on an already nose heavy car, do not expect it to handle well. It will likely turn-in harder but the car will pivot around the front nose more, causing the unloaded ass end to want to keep pivoting so much that the car oversteers even more wildly than normal. Also, it won't put down any power coming out of a turn (especially if you still have rear UCAs) because your rear geometry will be even more messed up.
If you have rear coil overs, you can lower the rear spring height to help move some of the weight back to the rear of the car back, thereby relieving some of the problems of taller rear tires (or, with traditional springs, just cut them). You can also try tuning with sway bars, shocks, spring rates, etc..
Are there fast track cars out there with mismatched front/rear tire sizes? Without a doubt. Would they go same sizes front/rear if they could? Probably, imo, unless they've got some trick front spindles/a-arm/k-member setup which addresses the problem.
Last edited by aurdraco; 09-30-2009 at 07:36 PM.