Well FWIW, I've always fed my GTI on SUL, but had to resort to NUL lat week for a tank
It didn't seem to make THAT much difference to the performance (but in all fairness, I was only driving to and from Borth - 150mille round trip, with the family, so I couldn't boot it)
However, my MPG got HAMMERED!!!!
The best I could get out of it was circa 27/28mpg. The same circumstances with SUL, would be another 10mpg (or thereabouts)
I won't be going back to NUL unless really pushed.
Dan