I've been looking at these as well. I'd like to get the OEM kit but I'm not positive it will fit the GTs. There are a few other options in carbon fiber, but the fitments also say DH only. MP Concepts has one, but I don't like the change in reverse light on theirs.
I believe the mp rear valence will use the factory reverse light bolt holes look the same. I did go with the mp valence as the dh does require as i was told different mounting points and bracketry which drives up the parts count and pricing. Should be able to make venting the rear cooler function or mod a dh under pan to fit. I iknow the reverse light kit that comes with the mp valence is the one they sell for replacing the factory one. Ill check the bracket that comes with it after work but i think it just extends the base gt belly pan to meet the dh styled one. So the factory dh pannel may fit or fit with minor mods.
I'd love to get your thoughts when you do install it. It might be the only option for now. The light doesn't look long and skinny like the stock light does and I'd prefer it not to be a brake light as well, but if it's all we have, I'll just have to bite the bullet. I'm sure you've seen them, but there are two underbelly pans that look identical, but have a different part number: KR3Z-6P013-A which appears to be for the S550s, and KR3Z-6P013-B that the DH uses. I honestly can't tell a difference and assume they would fit the MP valance just fine.
Hopefully i did see the a and b parts. For the rear belly pan. For now i will be doing the install with the base gt pan. Then getting the dh variant. If the factory light doesnt fit ill see if i can make a bracket or 3d print a part. Im not a fan of the flashy flashy brake light reverse light combo.
