[AW128i]

I'd say what you're feeling is normal. Perhaps you didn't notice it before the exhaust because you weren't paying as much attention to it? With the downshifting, I've noticed that the ECU will sort of rev-match in Steptronic mode. I think it's that, that causes a sort of freewheeling feeling, which can be mistaken for accelerating. It's not that the car is accelerating, it's just coasting briefly when you're expecting perhaps deceleration.

As for the upshifting, this is still a torque converter automatic so perhaps you're noticing the brief period of time when the transmission has upshifted but the torque converter is in the process of locking up (the TC is unlocked in-between shifts)? Maybe not, since that is a split-second thing.

Overall, you're probably more aware of what's going on because the engine load is now more distinguishable since the exhaust is enhancing the volume and tone of the engine.