This feeling of the power coming back on harder and more fully is (I'm fairly sure, anyway) false, I'm afraid - as GT4 automatically cuts the throttle when shifting up a gear anyway, you're not helping the engine by letting off yourself. I think the impression of getting more power coming back on is because the throttle was closed more and for longer, and so the car has started to decelerate more, and is leaning forwards more: so when you open the throttle again, it seems to accelerate harder, when in fact all it's doing is getting back to where it was before - just with a greater difference than if you'd kept the throttle open.
I hope that makes sense, and I hope it's right as well, but it certainly seems logical to me.
DE