the way the tdi is stock it has full boost at 1900 rpm. if you switch to the bigger one(passat ko3) you will loose the 1900 rpm full boost. it is going to be in the 2300-2500 range. the reason the tdi housing is smaller is because the redline is 4500 so they wanted boost earlier so you can have usable power, if you put the passat ko3 on you will run out of rpm before the full boost would be reached. in first gear this would show up as massive turbo lag. the higher gears would not be as noticable.