I run EP4200's charged at 6 amps. I also run CO27 motors. Your gearing is about right. Check with Casper over at the Banzai motors thread or on the Kyosho RB5 thread to be sure though. He is a fast Kyosho/stock driver.
The key to running stock is carrying speed through the corners. I set my car up to have quite a bit of roll so that when I let off the "gas" the car will keep rolling through the corners. I also get rid of alot of drag brake. I find that it slows down the car too much when entering a corner.