AvGas vs. Regular gas for Pawnee?

Synopsis from our chief tow pilot

"Per Howard Kron's current STC (which I bought and is our basis):
Higher compression pistons (but not actually THAT high; 90/96 or
100LL required); RPM max still 2575, same as 235; prop replaced due to
hub damage ..... which was not previously
picked up, and Factory New prop pitched to the original factory 84-52
(had been whacked back to ~49 pitch).

Net effect, much stronger, climbs like a raped ape. The AirCare
twicepipes (i.e., dual exhaust, NO!! mufflers) STC probably helps
significantly, too. In our operation, 65-68kts TAS is right at redline.
If anyone wants faster, they'll climb slower, owing to
too-flat AOT, and probably have to throttle back to stay legal."

I recall some discussion that the 250 STC is probably a better option
than the 260 (we couldn't find a core), as the 260 has a jump in fuel
burn for the extra 10hp. Apparently a lot of the 260hp cores are on
those air boats we all saw in Louisiana and Mississippi after Katrina,
with no maintenance records and a lot of swamp water exposure.

Frank Whiteley