When I bought my S40 (an '06) it had a 1/2" hole drilled in the end plate on each side of the normal muffler outlet courtesy of a previous owner. . It's still pretty quiet and I think it sounds fine.. Other than those holes, a Raptor petc#ock,and a Versy cam chain adjuster, the bike is completely stock, including the jets in the carb. The holes have had no noticeable effect on the way the bike has run for the almost 18K miles I've put on it since buying it.

The stock muffler can be modified to support significantly more power, and still remain quiet.  I was able to squeeze 41 HP out of an LS engine with a modified stock muffler.  It's a bit of work but the results are satisfying.

The stock muffler is complicated.  It's best to understand the beast before you start hacking on it.  Just drilling holes in the back plate doesn't do much.  This old post will show you exactly what's inside the beast and give you a good idea of the flow path.  It also gives you some good details on how to modify the thing without making the motorcycle sound like $h!t.  Good luck.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1526453881

BTW, that's a mighty clean lookin Savage ya picked up.  Congrats.

What cam were you using to make 41hp, DBM?

"What cam were you using to make 41hp, DBM?"

That particular engine used an OEM DR650 cam.  It was a unique setup.  The main point is that the stock muffler, with some relatively simple mods, could support 41 ponies.  It was quiet too.  If you are interested in the engine build you can find all the gory details here.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1567201633

I was always preoccupied with detonation on that particular engine.  I have since learned that the Savage combustion chamber is very forgiving.  I doubt that I ever had any detonation issues with that engine.  I was simply paranoid.