I don't know about a turbo motor but it seems to me the lower the RPM the better the MPG as long as the cam is right meaning little to no overlap, the exhaust allows raw fuel to go through the engine at high speeds because of scavenging and at low RPM it allows exhaust to go up the intake and dilute the air fuel mix so it don't burn properly. Also running the engine slower decreases the intake manifold vaccume because of the higher throttle angle, this means the pistons fight vaccume less (think about engine brakeing how the high vaccume slows you down). Higher compression and a later closeing intake valve can increase fuel atomization by forcing the raw air and fuel back and forth across the valve seat, also this allows a much higher expansion ratio that helps the engine operate efficently and lose less heat to a large combustion chamber. I think that would be a good thing in a turbo motor as long as you keep the dynamic compression ratio low for when you are under boost.