Well, 6x9s suck, so the difference you will see will be better sound. As far as mounting the tweeter anywhere, I would advise you to keep it close to your 6.5 inch driver. Keeping the midbass and tweeter close together helps with your staging and imaging. If you car is a coupe, think about adding an amp and amping your front speakers. Rear fill isn't something needed in a coupe, so like I said, possibly consider running an amp to your fronts and dropping the rear altogether.