Gear noise is a function of both contact ratio and accuracy. Helical gears are inherently higher contact ratio. But high precision, spur gears can be made to run pretty quiet as well. Lots of science to the deign of gear trains and the harmonics produced. Submarine propulsion boxes are the pinnacle of such design.
Regarding HP loss, consider that both Pro Stock and NASCAR use belt drives. I've seen the dyno results prove out. It isn't so much the belt its self, but the advantage of running dry with no losses to oil whipping.
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