Stum: I've been considering using my Tekno kit(coming in) and mounting the ESC like you since I'll have the mounting holes on top the motor. This would get rid of the issue of having to add wire inbetween the motor and esc. How did things balance out this way?
I used to have the esc on/behind the centerdiff, now on top of the steering servo. it's good to keep the center of gravity low, and weight to the front (better steering, less wheeling). much better.
pic of my "special" hyper7 conversion:
until I have the money for the losi exc I have this modded chinese-esc (great except the breaks).
florian