On these cars you typically feel boost start around 2000 rpm. It's all in by 2500 rpm. I measure about 5 psi at 2000 rpm so boost actually starts earlier, just not enough to feel it. The transition to boost is much smoother on new cars to the point that you don't feel the transition. But new was over 20 years ago If you're not up to installing a boost gauge, disconnect the boost signal line to the ALDA (squarish device sitting at an angle atop the injection pump, and see what the car feels like without boost. Use that as a baseline to determine when boost kicks in.

The engine is theoretically more efficient with the turbo. If you drive the car with the ALDA disconnected while applying exactly the same pedal pressure you would if the ALDA were connected (i.e., you don't mind accelerating more slowly), I doubt you'll see much change in mpg. If you try to make the car go as quickly without the turbo as it goes with the turbo, you'll burn more fuel.

Sixto
87 300D