domino, the guys at mercury fabricates their own downpipes. My r35 uses their downpipes and i think it's great.
When i got my downpipe install, i got it install with the y-pipes and exhaust pipes. At first the droning was kinda loud.
After driving for a while, the droning sound wasn't that loud anymore, instead it becomes a nicer sound to listen to while driving.
Best suggestion is to talk to Trent @ mercury. He should be able to answer all of your question