Never seen a Raggazon exhaust but there are others, and for less. I put a Remus on my 1.8 16v coupled to a Supersprint center (from Cat). There is no extra noise but it is deeper and hopefully will develop with constant thrashings. I have a one box in the center and obviously a rear silencer.
Noise evaluation depends on your perception. For instance I am thirty two and like a good sound. I don't want a boomy exhaust as I drive quite quickly (90 or so on motorways normally and just a little slower on quiet A roads). Also, bloody mortgage meant sale of other Alfas, so 155 must do my fiancee as well.
If I wanted to produce noise for attention, I would have used the rear box only, but high speeds would be a little uncomfortable. If I wanted to get pulled all the time, I would fit a group A as they are 'kin loud.
I suggest you try Alwyn Kershaw for a Supersprint system or Alfaowner or Elite for a Remus (backbox only). Both Remus and Supersprint are excellent quality and the top end of the market. They should last a few years due to finish and use of stainless on outer skins. Ansa is also available- try Demon Tweeks.
Sports backboxes are well worth it as they have proper absorption silencers (perforated pipe or straight through). The original on the 155 is a resonnator (silly arrangement of pipes and flutes with no obvious route for gas to take). A sports system will yield a little more power which should be barely noticed.
My Supersprint centre cost approx.£160 and the Remus back box would be more like £200- I got mine for a little less as I'm trade. The Remus tailpipe is a 135mm*75mm letterbox which is in keeping with the original. Supersprint tails are DTM (only if you have no taste) or Viper or twin round 70mm.
No bumper mods should be needed for a good system.
Ally- 1.8 16v