I adjusted the voltage via the castle link cable. You can go up to 9.6v if I remember correctly (not a good idea for servos, maybe a fan?). A little more voltage makes a big difference.
I have a single 645mg in my e-revo. It works ok, but a touch more torque would be nice. I was running two 645's for a while, and it worked very very well, but I wanted to go to one serve. At some point I will upgrade to something stouter.