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another thing that is a common misconception is the issue of drivetrain loss. it is NOT a fixed percentage. it is a measurable number, assuming you have a dyno that can measure the load. yes, the frictional losses increase with power output, but not in a direct percentage. these cars generally have between 30 and 40hp loss via the drivetrain. this will be affected by things like tire and wheel weight, flywheel weight, tire pressure, camber angle, yada yada.
frankly, calculating to the flywheel is pointless anyway, and only good for bragging rights. it's what you get at the wheels that matters.
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yeah, thrust at the contact patches is what pushes you down the F=ma.
Drivetrain frictional loss is dependent upon load f=Nu. Imagine dragging a brick across the floor, it takes a certain amount of force to overcome the friction between the brick and floor. If you double the load by stacking a 2nd brick on top of the 1st one, it will require more force to overcome the friction and slide the stack at the same speed as before.
A 500rwhp 951 will lose A LOT more power (70-80hp) through the drivetrain at full throttle compared to a stock 150bhp 944. Imagine you've grabbed the rear wheels and force it to stop. There will be a certain amount of grinding and friction between the gears, bearings, etc. The higher the power, the harder the load and the more grinding and friction you'll have between the parts.
With practically zero load with the wheels in the air, I can turn the crankshaft by hand in 4th gear and spin the rear wheels. I'm putting out nowhere near 30-40hp with my arm to overcome the drivetrain losses just to spin the crank once.