I was experiencing something similar - when I had to accelerate quickly while already at highway speeds, I'd get a sudden drop in RPMs. Speaking with some of my mechanic friends, I was told that if it was clutch slippage, the RPMs would spike, not drop - which makes sense since the engine would be able to rev freely if the clutch slipped.
However - since that discussion, my battery died and was completely dead for a few days. I've also had the fuel injection system cleaning service done and I haven't been able to reproduce the problem since. Not sure if this helps you or not.. good luck!
Note: I've got the HPA tune and downpipe in my car, along with the Haldex Sport controller.