There is a company in the USA who is doing interesting Dyno testing on a M117 560.
https://www.facebook.com/Carobuengineering/
This is an average mileage USA spec engine (same as AU).
The first thing they noticed was that they needed to recondition the heads as the valve guides were worn and problematic for good power.
Next they compared the USA/AUS spec exhaust system. They found the Tri-Y system to be 11% more power than the USA/AUS spec.
They also looked at the...