First of all, I own a late model Mustang and the Trans Cammer was one of the first cars I bought.
Secondly, one of the biggest rewards you'll get out of any of the GT series is figuring out how to dial in a car like this. All on your own.
Lastly, have you tried a Cobra set up in it? Why would you put sport tires on a race car? You could try to dial it in on sport soft tires I guess. I'll assume you are dealing with a really loose car and can not control the rear end. Start by making the rear slightly lower and softer.
In closing, when the time comes that I have some free time and feel like spending days trying to figure out this car I will. I won't come here and ask for a set up and in the end that will be my reward. A little more knowledge about taming the beast. After all, what would you do if there was no internet.
I'm not trying to get after you. Heck, I failed algebra twice and it was not until I had to figure it out myself w/o the help of others that I finally got it. See where I'm coming from.