Castle motors are completely different so it's hard to compare. I have the 4600, the 5700, and I sold my 6900 because there's no need for it. My 5700 is as controllable as a 10.5 on the bottom and as fast as a 4.5 on top if geared right. I've actually won a lot of races in my 4wd with a 4600, where a 10.5 wouldn't have even came close to the same performance. Read the tuning guide on Castle's website and you'll get an idea of what I'm talking about. When they say you can just gear up for more speed, they're not kidding.
What esc are you running?