Well, I don't have a C50 but when I started getting into brushless this was the only motors available. Feigao were not out yet (at least I didn't see them). Anyway, here's some info on performance from the hacker website it you didn't see it already:
http://www.hackerbrushless.com/cars/otd/emax_otd.shtml
I don't think you'll be disapointed because you didn't get a 9L etc., You could always save up and buy one, they are way cheaper than hacker. I believe you need the 72t spur because the way the motor mounts are, it doesn't allow you to adjust the motor any closer, so if you got a smaller spur, the pinion would not reach it properly (correct me if I'm wrong).
You should be happy going to brushless though
patch