Running higher octane won't prevent fouling. If anything, it will promote it over the long term, as higher octane = harder to burn.
I don't know what sort of sound is expected from a GT's gas cap, but the fuel system works pretty much the same way my RS's does. Turn the key and the fuel pump pressurizes it within the first five seconds or so. If you put your ear to the cap immediately after the pump shuts off, you should be able to hear it if it's losing pressure.
I wonder if you might not have changed your readout from UK gallons to US. Are you looking at the computer, or doing the trip meter vs. gas pump thing?