I agree with the comments about big brakes on the back. Due to the loss of grip resulting from the weight transfer, there's only so much braking power you can effectively apply. Even a 160mm will provide plenty of power.
That being said, I run a 180mm on the back, but that's because the bike came with it.
As for the front, I can testify that a stock Formula RX set with 180mm disc (around £150 for a new complete set, front and back) will happily throw you over the handlebar at whatever speed you can achieve.
I really don't see the need for more powerful brakes.