RE: What to do with k&b,kbv ducted fan motors?
If you go with that idea, make sure the airfoil is optimized for speed. Even if you have to utilize some basic modeling skillz [OMG, he means actual work..!!] to make that happen.
Eliminate any other major "drag brakes" like large expanses of firewall hanging out behind the engine and make a cowl that blends into a bullet shaped spinner.
All the little drag reducing details add up to a greater result until the basic design "hits a wall", then it's time to go back to the drawing board in search of a better design to make best use of your powerplant.