By definition, "Power" is a measure of Watts which is calculated (using Ohm's Law) by multiplying Volts times Amps to get Watts:
https://www.wikihow.com/Calculate-Wattage
By definition, "Horse Power" is calculated by multiplying Torque times RPM:
Power and Torque: Understanding the Relationship Between the Two, by EPI Inc.
For tuning mechanical timing on the end bell for stock racing, most manufacturers will provide a recommended current draw at 8.4V for use with a motor analyzer like this one here:
https://www.rcjuice.com/skyrc-brushl...r-checker.html
For example, I run a Trinity motor in 13.5T stock buggy and they recommend setting the timing to 5.6A draw. The digital timing takes an average across 3 phases measured independently and they almost never match up to what the reading on the end bell says regardless of the brand of motor. Anyway, you want to periodically check/adjust your timing as the end bell can shift in a crash and magnetic strength will fade over time which will require you to adjust the timing to get max optimal power
draw without wasting energy/heat.
If you want to find optimal gearing in addition to optimal timing, then you would want to invest in a dyno similar to this product here:
https://www.minipro.com/products/chassis-dyno