Peter
I guess it will have some impact, as BMW have clearly gone to a lot of design/cost input for the next model Inlet ie your M52 version with the special chamber for low revs torque. The great thing from my point of view is I'll never notice the difference as I never drove the car in its original state!!
I could get it rolling road tested if I really wanted to, but that's a long way off yet............
Have you investigated how the M52 will sit in your Sportster engine bay across the width? I know Andrew Curtis had problems with the M50 even though the Sportster is wider than the Cabrio. He chose to make a special glass fibre elbow:
I suspect this compromises the full volume flow at peak power, and is more likely to have an impact to the peak power, which is why I chose the route I took. I am guessing that if I have changed the characteristics of mine it will be at the low revs end, but that is only my gut feel?
Mike