I get about 23mpg out of my NA, that's driving mostly like a grandad (flat cap is in the glove compartment!), with the occasional Heavy Right Foot on most journeys. That's also with a clogged air filter, and knackered spark plugs
Not sure about the theory behind getting better MPG from a TT. If you stay off the boost, then possibly..... But with boost, it seems like getting something (i.e. better performance) for nothing (no extra fuel used, or even less fuel used), and something for nothing doesn't usually hold true, IMHO.
Hope that helps a bit.