The sidewinder may not be able to handle the feigao 8s, as it uses more power than a 5700 which is the limit. kvs are misleading when comparing motors with different size rotors. You could gear the 8s the same as you would gear a 4600 for the same top speed, but the 4600 would still be more efficent. Although you can argue that you would get more torque out of an 8s that way, why not get a 5700 and gear it down to get that torque.
Check out my custom converted 8ight:
Sub 7lb, lowest CG of any 1/8 buggy
I would say to go with the 8s feigao. I have found that the mm motors are only good for lighter weight 2wd 10th scale vehicles. The Feigao s can motors can draw some power, but should run cooler on the load you have.
I know that rchippie runs a 7s motor on 2s lipo in a crt.5 (I sold it to him!) and likes it. Seeing that thew Cen vehicle is rather heavy the 8s would be a good choice, for 3s lipo.
I have had all of the mm motors, and think that they are overpriced, but they do sell well, most likely due to people knowing no better! So I have sold all of them!