I had a custom made Y-pipe installed on my Stillen Maxima well before Stillen ever released one. It took the shop about 5hrs to fabricate the new pipe out of mandrel bent steel. I had it dynoed before and after...+19HP to the wheels. No doubt the Y-pipe worked harmonously with the cold air induction and exhaust already on the car.
Yes, mass produced pipes hit the market for half the price I paid, but that was a full year later!! As long as you are confident in the shop's machining and workmanship, I say go for it. If you are in question about that, then wait for the commercial ones to hit the market...this is an M5, not a Maxima.
As far as noise, it will certainly be louder, especially if the stock Y is routed for silencing. If that's the situation, then the gains will be good as a result. (this was the case with my Nissan)