My concern would be that the obvious way to improve fuel economy is to run a leaner mixture (less fuel in = less fuel burnt), but that also means running hotter with the risk of more DET. - Obviously I'm not suggesting that the person mapping your car doesn't know that, nor that when there's no load on the engine that doesn't make a lot of difference.
I'm just cautious about the prospect of making a map lean about where you'd be cruising just so that you use less fuel; maybe it's just an aesthetic thing, and I like the idea of nice smooth fuelling curves rather than wobbly lines. Does that also give you a really odd power curve/delivery?
It seems to me like there's already a sweet spot where you're not making boost, but the turbo is no-longer acting parasitically; a balance point where the fuel economy seems to be best.
Ultimately this isn't a car that was designed to be cruising on motorways being really fuel efficient; it was designed to explode out of corners and hold on to loose surfaces. If you want fuel efficient, just buy a hybrid; as long as someone is supporting the technology it will mature and become better with each iteration.