Is this gonna be the 550GTS or the 575GTC? As far as I can see they overlap in terms of active years around 2003-2004!? Are they related or is the 575GTC completely different?
The 550 and 575 were different, on the road car side of things the 575 was the 550's "replacement", Ferrari changed a few things and increased the displacement hence the name change. You can tell them apart by the fact the 575 is more angular than the 550, especially visible around the front head light.
The thing with the 550 GTS and 575 GTC in terms of racing (besides the fact the 550 was built by Prodrive and the 575 by Ferrari) was it came out at just the right time, especially in FIA GT. By the time the 550 arrived there the Viper were well past their prime and survived only by reliability, the Strom was reaching it's end, the Lambo was made of glass and the Saleen was the only really viable competition when they were reliable.
If you look back you'll find the Ferrari was the best of the litter even in 2002 in FIA GT, though the Labre Viper won the championship thanks to winning the 24 hours of Spa. From then on the 550 was the car no one could beat. It was a mature car being actively developed for endurance racing (Le Mans) by people who knew what they were doing and had the experience making reliable race cars built like tanks.
Compare that to the 575, which was built by people who clearly didn't have the same experience or make the same effort in making a V12 GT car competitive at the time and came out a few years earlier - by the time it was mature the DBR9 (Prodrive's project after the 550s) was out, the C6R was coming, and the Saleen was still around. Oh, and the MC12 cheater-mobile with all the drama around the rear wing.
Yes, I threw shade at Ferrari's GT racing program. I still think if it weren't for the Prodrive 550's success they wouldn't have built a 575 race car. I also don't think Ferrari was that active in developing the 575 beyond just making it while Prodrive made more effort in order to keep up with the Corvettes at Le Mans.
So they are two different cars, one was successful, the other comparatively wasn't - but there may be reasons. What a difference a year or two can make.
This is why GTR2 is so great, those two years of 2003-2004 perfectly capture the rise and fall of the old guard in the Vipers and Storms versus the new blood with the 550s, Saleens and the Lambos.