I've found the XJR-9 to be in the championship roughly 7 out of every 10 times I reloaded it. It's common but not that hard to get rid of. Of course you'd know that if you read my first post or tried it for yourself a couple times. Gthead01's post seems pretty clear as well("Yes, it was the A-spec Dream Car Champ").
Just load it, look for the XJR-9, if it's there, go check the UCD, then reload it. It won't be there soon enough. You can scan for it in just a second or two, as there is rarely if ever another purple car. The XJ220 is irrelevant(so don't just look for Jaguars like I did the first couple of times

).
There is no other real rabbit-car as far as I know. The Zonda is fastest but it's not even close to a prototype, and the Ford GTs can give it trouble enough to steal a couple wins or hold it up for you.