Am I missing something

It's kind of easy to throw out the 25-30 number. And in some cases it might be true. And can be generally close to accurate.

The reality is it's MUCH more difficult than that to figure out exactly the power loss through the drive train. As it is a curve, not a straight line graph. IE 400HP400TQ engine doesn't just automatically lose 100HP/100TQ just because it has a 727 and 8-3/4 behind it and a 500HP/500TQ doesn't just automatically lose 125HP/125TQ because its not a straight line. A 400HP/400TQ might lose 25% or 500HP/500TQ might lose 20%. But other engines might lose more or less. This is the problem with the whole "727's take X amount of HP" argument as well. Essentially it's a curve that depends on all the inputs (engine power, torque, where it makes it, transmission, transmission fluid, changes to transmissions, bands, rear gears, rear gear oil, etc etc etc). It's basically endless. It's similar to how we spec hydraulic motors at my work. A guy says "I want the motor to do 400 rpm and 20 ftlbs" so we say "Ok whats your input GPM and PSI" Guy says "Doesn't matter, just has to do 400 RPM and 20ftlbs". Except that it does matter. Every motor's output is dependent on the input. It's the same with the drive train loss. And with the almost endless combinations out there, it would be very difficult/next to impossible to come up with a general graph to show what you can expect. Basically. Build the motor you want. Worry about numbers later.

If my explanation makes any sense.