I have a Powerflow on my 155. It was noisier when I first got it but has quietened down. I too do a lot of motorway driving, but there is very little noise unless you accelerate. At idle I have a low rumble which transforms to the nice "Alfa" rasp at higher revs.
With regards to bore size, mine was fitted with the same bore as the original exhaust. I also removed the corner box to give only 2 silencers instead of 3. They also built me a stainless decat using a small box (free of charge !!!) as when they tested the car, it was a lot louder than normal with the straight through decat pipe I had on.
Going back to bore size, you are best to keep as close to the original unless you are running a turbo, it's all to do with "pulse tuning". A wider bore lets the gas pulse cool down, therefore becomes heavier, doesn't move as fast and doesn't create as much vacuum behind it to "pull" out the next pulse. Make sense ????
As someone said earlier, the standard of Powerflow centres seems to vary. The garage I used prepare cars for track/race use and do a lot of work with Scoobys, Evos and other very tuneable cars.