right, both are brushless and the outside is the same size.....but the one you bought is a smaller rotor on the inside.
Its possible your radio calibration is off and its not going full throttle. try recalibrating.
Can you confirm the pinion gears are the same size? The original brushed motor may have used a smaller pinion than the epx pro does. that would make a difference. even 2 teeth less will result in a good 3-5mph slower.