which motor for speed on 3s lipo

I'm going to assume that you are using a Mamba Max esc. The 5700 would be somewhat of an upgrade. It's simply by virtue of the fact that the 5700 turns more rpms per volt than the feigao motor and the 5700 has the torque to be geared. On 3S lipo, assuming 3.5v per cell under load, the feigao will turn about 51,500 rpms. The mamba motor will turn about 60,000 rpms on the same voltage. Are you running this in a 1/10th scale? The fastest readily available system is probably a Mamba 4600 on 4S.