Hey guys! There seem to be number of members that are interested in this topic so I thought that I will chime in.
I'm the lucky one who got the 330i 3-Stage manifold from RichP. The installation was relatively easy. The car runs basically the same, maybe little more low end torque, but top end is the same. As a side note, I'm not one of the guys that can feel a SOTP difference from an air filter...
However because the car runs without problem, it's my belief that 325i ECU has some control over the manifold "straight from the box". If the manifold would be stuck in the stage low or medium RPM stage you would definitely loose some top end power. My perception of little stronger low end, does not lead me thinking it would be stuck in the high RPM mode either.
However you need the 330i programming to get any benefit from this mod, otherwise it's useless. And that's the tricky part. Evotech (trough Creative) said that they can flash my ECU with their 330i SW. By now they have had the ECU for 3 weeks (was supposed to take a week) and I'm about to loose my faith on them. I hope they prove me wrong...