Knock sensors can make a big difference in the performance "feel" of an engine. Not sure about the later VWs, but the knock sensor system from the 1985-1988? A-2 Chassis GTI and GLI worked by constantly advancing the ignition timing like 1° at a time until the knock sensor picked up detonation, then it would instantly retard the timing like 3° and start the cycle again. In theory the better the fuel, the more advance you'd get and at a faster rate. I used to swap that system into low compression (8.5 to 1) engines and that change alone would make a big difference in the "feel" of the car. Not sure about the later systems, but a bad knock sensor (the mounting bolt torque was critical) would take that all back, especially if it was bad enough to trigger limp home mode. If memory serves me right, Chevron or Shell gas gave the best "feel".

In Florida, ethanol free fuel is pretty easy to find and in my area usually less than $1 extra.

I can't see ethanol eating away the plastic body of the fuel tank. I can see it ruining an O ring for the sender or eating away a rubber vent hose.

As I said, the plastic is like just old. Picked up an older disposable food container, and it shattered with a light squeeze.