premium gas is not entirely necessary unless you runn higher boost or drive in hotter conditions. you could go with mid grade gas as far as the overall cost of fuel is concerned
probably the best thing you can do for highway speed fuel economy is go with a more open exhaust (after the downpipe) the turbo will work more efficiently, and you wont be into the turbo as much to get going if the exhaust can breathe easier.
driving habit will play a major role as well. drive at a lower average speed, or ease into a higher cruising speed.
if you see a red light coming, time it o that you slow down enough but have not stopped by the time it turns green, things lime that.
inflate the tires to within 10 % of the max peressure, for example, a 44 psi tire should be aired to 39.6, or a 35 psi tire at 31.5 (essentially 32psi)