I think you are going to have a hard time flying with a 5" prop. With a 2700kv motor you should start testing props around a 10x6 or 10x8 and go from there. The best way to tell amp draw is to get a power meter or amp meter that will go up to like 40 amps. Are you trying to justify going with a smaller prop because your motor is bigger than the recommendations? Also, I am not completely sure that a 25amp ESC will handle that motor to begin with. What is the amp range of the motor, both contiuous and burst?