For us, I'd be willing to bet MTX nets you about 15% loss. The reason I say this is my 122.9 puts me at about 144.6 at the crank. 20% would equal 153.6, which I seriously doubt I'm getting.
reaper4tour dynoed at 123 on an ATX Duratec...a 27.6% loss. Running an MTX, he likely would've gotten around 144.5, assuming 15% loss.
As with anything, this isn't written in stone...just my thoughts after some observations.