This is kind of interesting:
http://www.superstreetonline.com/how-to/engine/modp-1005-drivetrain-power-loss/TL;DR: The 15% "rule" does not hold true at all and varies widely. A more powerful engine will experience a higher HP loss on a specific drivetrain due higher acceleration (dynamic drivetrain loss.)
I would guess our little 22RE platforms lose way more than 15%. Our drivetrains are built (over-built) for reliability, not efficiency.