Hi Steve,
I can confirm that my 2¾ (1913 TT model) returns about 80-85 mpg on the Pioneer Run and is capable of about 60mph in 'level flight'. The engine was fitted with modified postwar pistons some time back in the 1970's. Other than that, and a big cam, the engine appears to be pretty standard and has now done thousands of miles in the hands of it's various owners (I have the full history of the bike). The main criteria when rebuilding the engine is not to over-engineer it! Being a splash fed oiling system, the bigends need enough slack to let the oil in (say 3thou clearance) and pay particular attention to getting the bronze shells correctly fitted so that they are trapped by the keep plates while the conrod cap seats properly on the end of the rod. If the cap (and keep plates) is just sitting on the ends of the shell bearing, it will eventually work loose due to the different rate of expansion of the bronze.
When you have the bike up and running - and bedded in, don't be afraid to give the engine it's head. The 2¾ engine is a very free revving unit and is much happier just buzzing along than being made to slog.
Regards,
Eddie.