I was just about to say, I will be transferring my GT5P garage to GT5 but only to use the cars in Arcade mode, if I feel I need a break from GT Mode.
Will not transfer any credits over though, GT Mode is to be played properly IMO.
It's the perfect solution IMO, you can play around and have fun in Arcade Mode with any car you want while still able to experience GT Mode the way PD want you to.