I regularly run thundertiger .40s (basic $70 motor) at 16500-17000 with APC 9x6 props. 15-16000 is not an unreasonable range for many .40ish engines; some are fine at 11-13000 and others run better with lighter props. It's worth checking the manufacturer's recommended RPM range when you choose your prop, as some run really poorly when overpropped.