From carb 1.3 to EFI 2.0, don't expect to improve on fuel mileage. In fact, prepare to spend more.

As for the GTi, it shouldn't be much more of a guzzler than the MPI. They're both 2.0 4-cylinder anyway, so the difference isn't so big. GTi is a local classic while the MPI is just your regular Galant (like Civic SiR vs. VTi). If you can afford the GTi, get this one instead. AFAIK there's a small club for this car which should be fun to join.