better traction, rear wheels, and ETC

I've only had my 07 Gt for about a month, and I've already found the limit of my stock 235/55R17 Pirelli tires. On a cold day, in a completely stock car, punching it in first results in spinning the tires from 3k on up.

I'm interested in not only getting wider rear tires, but also bumping up to 18's in the back, for a few reasons. First, the added size in the back will help with the look (slightly old school, bigger in the rear), but also with more options as far as rim size compared to 17 inch wheels. As much as people complain about it, and as much as I check my tire pressure, for some reason I kinda like the tire pressure monitoring.... it just seems like a nice feature.

So who here has replaced their rear wheels with a different size than the front, and did it cause an issue with the TPMS? I guess that in the end I could choose a rear tire that would have the same effective outside diameter as the front tire, which would not affect the pressure system.... but I don't want to be constrained to that. I'm just looking for a wider rear tire, with good traction, that doesn't require changing all 4 wheels.

P.S... On a somewhat related note.... what is the widest rear tire that fits on the 07 GT without other mods? I'm not too concerned about backspacing as long as it fits mostly under the fender flare. This could influence my decision on what to do... I'm just ready to upgrade after dealing with these pirelli's.

Thanks for the replies all, but what I'm really wondering is if the TPM light will come on, even if I do switch the sensors over.

The way I understand it, the computer reads the rotational velocity of each sensor, and if one is slower than the rest, it will turn on the light. This is because a flat tire will rotate slower than an inflated tire. If I put 18's on the back, and leave the 17's in front, this would make the rear tires turn slower than the front.

In theory, I could buy front tires so that the total overall outside diameter of each tire (regardless of wheel size) is the same. But again, that is a pain, and I'd rather not have to do that.

So is there a way to program the tire pressure monitor for larger wheels in the rear than the front?