Mustang - Cool render. I don't think the XRR is too big, but I think that huge spoiler makes it look big + makes it look a bit crappy.
iFastLT - Awesome render and also the rim. Have you made any renders where that rim is on a car? If yes, can you show me, if no, make one.
I think it will look good on a car, but maybe I have even seen if you have made, but just forgot by now.