Opinions please..mild +t for economy

[ndst245993]

Sorry about the lack of introduction and such. I just had one quick question for the community. I'm Neil by the way, and the car in question is my girlfriend's 08' hatchback.

was wondering if a mild turbo setup (thinking t3/t4 with a mbc, in the 3-5psi range), driven very gently, by an eco-conscious girlfriend, would return better, worse or comparable fuel milage. I have seen online, performance applications of this motor with forced induction, so I assume that it can take a little boost, but no one that I can find has tried a +t for economy.
Keeping in mind that she Never puts her foot into it, and is conscious of every method "economical" driving. (read, I never drive it).

well, thank you all in advance for your time, and thoughts.
Cheers,
Neil