Alibob, to your question about the Powerflow exhausts.
From peoples experiences I've read on 155.com, the results seem to differ from garage to garage. I read about someone on this site with a 145 said his was rubbish and too loud at motorway cruising.
I have one on my 155 and I've found it very good and very well built. Get a deep burble, then "Alfa" rasp at high revs when booting it, yet is extremely quiet at light throttle. I opted for a 2 box (standard is 3 boxes on the 155) "close to OE sound" system. Basically they used the same diameter pipe as standard. If you do a bit of reading on "pulse tuning", you may agree with me that going for a wider bore pipe is not the way to go.
The garage who did my zorst also built me a stainless decat for free, as the straight through pipe I had made the noise too loud for what they consider "normal" noise level.
Hope this helps