The one thing missed by the guy saying to derate RPM by half is that, in FW's especially, vastly overspeccing a motors power is the norm.

The common NTM Propdrive 50-60 or 50-50 used in FW drives are motors hitting into ~1-2.5kw per motor, well over what's really needed for a feather. The advantage of this though is that such larger brushless motors come with the bonus of rotating slower. May seem an odd advantage but when you are making your own gearboxes which is often the case slower motors can let you get away with simpler single stage spur gears to do the reduction. So in this case you can reasonably assume the rated kv is not far off the kv you actually get. With brushless motors the 3 main considerations are power (You want to over spec enough that startup torque is not much of an issue), kv/RPM (Ease of making gear reduction), Build of motor (Are they enough motor to cope with the heat generated, plenty of small motors would work... just briefly).

For a bit of inspiration these are the setups I made for my feather. Very robust, fast and powerful. Have even taken a couple of direct hits from Nuts 2 and carried on (albeit with a slight twist to one now!).



Of course there are other paths to brushless drive, such as more capable ESC's paired with smaller motors and gearboxes. I just tend to this one for feathers for reliability and cost.