Why would the turbo need backpressure? just see it as a big fan that you blow onto, does it spin faster when you block the airflow behind the fan?
I don't think so.
It's the same principal as turbo gas motors. I have a turbo 1.8L miata and a 2.5" is better than a 3.0". The reason it's better isn't because it makes more power - it's because you don't get any power gain going to 3.0. So the 2.5 is the same power, plus it's cheaper to make, it's easier to fit and less likely to bang on things.