I now have the Remus stainless headers and the Titanium Haxacone muffler on my bike, and am very happy with the combination.
I used it for a day with the 'db killer' baller still in place, and it was as quiet as the stock system, then once I took the little baffle out it became louder, but not too loud. It is a pretty perfect volume, authoritative under power but not intrusive or tiring while cruising. I think it looks a lot more elegant than any of the other pipes I've seen on the MM, and IMHO really suits the style of the bike very well. The complete system is also around 12lbs lighter.
If it made any difference to the power it is too subtle to tell from the seat, so the advantages are weight, style and sound departments.
Now after a 1000 miles I can 100% recommend this upgrade to anyone.