Regarding B spec or GT spec tires:
Getting the stiffer extra ply sidewall tires will probably not result in
any increase in mileage. At least this has always been the case with the Michelin Pilot Road line, and was also true of the "E spec" Pirelli Diablo Stradas.
Personally, I'm not sold on the benefits of these more expensive skins
unless you will be overloading the regular spec tire. Even though I ride 2-up about 50% of my miles, my passenger is relativity small (4'12" and ~105 lbs), so even though I qualify as a Clydesdale (6'2" 230 lbs) together we are a fairly normal load for the bike. If you get the regular spec tires and keep them inflates to a higher pressure when fully loaded (~40F / 42R) you'll get good mileage.
Now if both you and your passenger weigh as much as me... well maybe that B spec is in your future.