I'm having the same thoughts, on my A4 I've got 19's that are 9j on the front and 9.5j on the back if that's the correct way to put it (not up with all the wheels diameter phrases) running 235/35/19 tyres on both. Which means the rears are Pretty stretched which I'm not a big fan of, it wouldn't hurt to put some say 255/35/19 tyres on the back and leave the fronts as they are would it!!??