It's not that the stainless it's self is any amount heavier than regular steel, but I think a workable thickness that would be resistant enough to cracking and realistically weldable would add some weight. A lot more difficult metal to work with, but I did see a bit of scrap stainless sheet at the junkyard, I may just have to pick it up and try it.
Scott, I think it might have an edge on just a muffler, but with too much volume in the header, the exhaust would tend to mix up with the fresh charge, so you would be pumping both exhaust and fresh charge back in with the pipe effect, also the wave action would be messed up at the header and pulse strength lost. Now if the bolt on piece was made to align well with the exhaust flange, that would help. Also with the back of the muffler in place, it is going to be tight to make the turn without the pipe hanging out infront of the saw.