Ahh, but that would assume that the vanagon exhaust is optimized.  A
classic example of VW engineers' work is the 84 GTI exhaust manifold....10
HP is available by swapping it for an older dual outlet manifold.
Additionally, crush bending as opposed to mandrel bending is routine for
stock exhaust...and well documented for its effect on performance.

For that matter, if VW did such a great job...why are we all afraid of
blowing engines on our next trip?

I would agree that the Cooper S is probably already well engineered.

Perhaps the real test is for someone to:

1.  Check pressure readings on the exhaust under operating conditions.
2.  Try a different setup with a free flowing exhaust, and test back to
back.  I'd put my money on HP being available.