interesting topic. i am just a hack at rc planes just started less than a year ago. so i dont really know what im talking about. but after reading his original question. i cant help but wonder if he is talking about as an example:
you took his 8x8 prop and put it on a motor. the motor was just vice gripped to a saw horse and you were measuring thrust at different rpms

would there be a point where the thrust would decrease or stayed the same the faster the motor spun?

fake made up numbers here

15000rpms = lets say 61 grams of thrust
15500 rpms = 65 grams of thrust
15001 rpms = 64 grams
16000 rpms = 61 etc.
so the peak ammount of thrust would be 15500 with that prop
so i guess if he had 3 different motors to choose from 10000rpm motor, 15000rpm motor, 16000rpm motor. if the maximum thrust for that prop was at arround 15500 would it be worth it to buy a 16000+ rpm motor? or will the thrust decrease at all?
i know the whole thing is more complicated than this but (i hope) i translated his question geared more to the answer he was looking for.
crankypants