I think the M4 makes a great FPV machine and the 250 size is perfect. I have 2 flying with Kiss 18a esc's one on a scratch built frame and another on the HK FPV250 frame. The scratch built one uses a KX2 camera with micro servo and a Fpvhobby 5.8ghz TX. I use the MinumOSD for 5 volt power for the M4, video camera and the TX. It weighs 400grams AUW, the HK machine weighs 500 grams. Both have Emax Mt1806 2280kv motors.
I've had the H version go over 26 m/s with all the FPV junk removed and 3 cells. That's probably not the limit but I had limited space and to be honest I couldn't see the little thing. You almost need to be FPV to fly it full throttle or it gets out of view in seconds.

