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How to optimize engine performance?

Not sure how to phrase this, but... what do you think determines fuel
efficiency in a turbo? My 200QTW has very little torque at 60 mph; if I try
to accelerate, the boost goes up but the car doesn’t jump (accelerate)
quickly. Obviously, if I drop it to 3rd, it jumps. Additionally, the gas
mileage computer doesn’t seem to care if I am at 4000 rpm in third or 1500
rpm in 5th. In fact, it seems that “lugging” the engine (using 5th a lot)
actually DECREASES gas mileage because it INCREASES BOOST for a longer time.
On winding, twisty roads, gas mileage does not seem to suffer, even if I have
it in 2nd or 3rd, revving the hell out of it, for long periods of time.
Bottom line: Is it safe to assume that it is “better” to drive agressively
(in a lower gear) because gas mileage is the same or potentially better, you
have more passing power and more fun? What causes “engine wear” --running at
higher RPM or running at lower RPM but making it work harder at lower RPM? 
Empirically speaking does: Pedal pressure =Boost Increase = More gas used. 
Higher RPM = better efficiency?

With my ‘86 coupe, which had shorter gearing, my gas mileage always suffered
if I drove “agressively”--using lower gears a lot. If I kept it in 5th, gas
mileage was better. ...wonderful little car that was...could roll away from a
Stop sign (California Stop) in third with no lugging...

Thanks, Twain