Your theory is correct, but if you think the OEM calibrators left any efficiency on the table you're on drugs
![Smiley](http://nefariousmotorsports.com/forum/Smileys/default/smiley.gif)
If they've tuned for the lowest common fuel, there probably is a bit to be had if running on a better fuel. That's engine efficiency though, doesn't take the extra cost into account.
My Mk6 R is averaging about 5mpg (imperial) more on the commute with the same roads, same traffic and (unfortunately) same shit cold weather. Just with cam and ignition advance.