The issue with a drastically over-sized drive is controllability, especially at lower speeds. Accuracy will suffer, because the drive thyristor firing angle will be so low that it will be difficult for the drive software to manage. And if you are concerned about motor protection you will have very little because the normal operating ranges of the motor protection parameters (overload, over-current, etc.) in a 15hp drive are centered on 15hp motor parameters; they are ranges, of course, but you’ll be way out on the low end of these ranges and response will suffer.