By all means use what you have - an electronic trans with an adapter ans controller will start at $3k

As far as "percentage of power", consider that the engine only makes ~20 gross HP idling down the street, you can't have negative net power, so loss is not fixed.
Also, gear oils heat up a lot proportional to power/RPM and the 727 thrust bearing makes a lot of heat and proportional to torque (gear thrust) - like 10% of the waste heat.
The 8HP** uses a very thin Lifeguard 8 fluid to reduce its viscosity heating as well.

And in pure physics, power is equal to units of heat per second: 745 watt, is equivalent to 745 joule per second == 1HP