We had it done as part of a group buy a couple of years ago and we got a great deal. I don't know how much they go for these days but it might be worth mentioning SportsMaserati to the guys to see if they can do a deal for you.
The report on my GS afterwards was a 40 BHP increase on the standard 400BHP...10%..?? I have the original figures, but as it was a special deal it would not be fair to publish that on here and the figures will be out of date by a couple of years..!!
Mine only got a 23bhp increase at DMS but it was starting from a very healthy figure. That was probably due to being properly looked after when it was new, Larini sports cats and straight through rears. The spread of before and afters was very marked.
If you can get a 5%+ increase in BHP, then it's worth it, if you are only going to get say 5 or 6 BHP increase then it's not, I'm impressed with mine so far..!!
My version of the X pipe... I went to the local exhaust shop. They do normal and custom work. I showed them a picture of what I wanted. They ordered an X pipe kit and built this X pipe on my 2004 coupe. I think it looks and sounds great. $475, not bad for a Maserati upgrade.