Plain atmegas have been running thousands of tri,quad, octo, etc copters just fine, doing waypoint navigation, sensor fusion, inertial calcs, reading the PPM values from the RC transmitters, generating PPM for the ESC's, even data-logging and talking down to a ground station, all that in a 8bit micro running at 16Mhz with crappy Arduino libs that slow everything down.
I understand the need for better and faster, so why not a cortex M4 that has hardware FPU?