hi guys,
I agree with your demonstration but the formula you give only and your demonstration only allow to compare atmospheric engine between them. you forget a little detail which change all.
the YS 4stroke are supercharged and in this case the effective volume of mixture pumped to be burned in the cylender is greater than the volume svept of the engine .
in the YS 4stroke engine the supercharging effect is achieve by to systeme :
first : it is the rotary valve which produce the overpressure in the cylinder during the pumping but it is not enough
secondly : the fuel and especially nitromethane allows to give more oxygen in order to supercharge the engine, it is for that reason that people use 30% of nitro in the fuel

for example if you suppress nitromethane in the fuel, there is not any interest to use an YS instead of others engines running without nitro fuel.