I installed the RB complete front and rear setup last year with 400s on the front and I am EXTREMELY happy with the way they work. There is probably no "real necessity" to do the rears, especially if you only put 390s on the front, but I think the extra grab on the back helps to keep the car balanced, flat and straight under heavy braking. I may be a little weird, but I think really good brakes and suspension are more important for a great driving experience than a few extra horsepower. Not that there is anything wrong with more power...