I've got a Supertrapp 4" 'race' series on a KLR650. It came with my bike when I bought it used and it was very black on the end and sounded horrible.
I finally tore it off the bike and yanked the core to check out the packing and the discs...it only had three in it. It turned out that there was NO packing in it at all, of the three discs, the previous owner actually tack welded a solid center to the disc that was closest to the core. Basically the thing could barely breathe.
I upped it to 9 discs including a heat shield, and repacked it by using a roll of industrial stainless steel wool in fine grade. I wrapped the core until it would just fit back into the can, safety wired the wool in about four places with stainless safety wire, sprayed it lightly with WD40 and got the can reassembled.
It runs great now with the stock jetting, the pipe is at least half the weight of the stock KLR pipe, and the sound with the new stainless wool packing is really nice. It's a warm low 'thump', hardly intrusive yet sounds really satisfying when WFO and running through the gears in traffic. I haven't put a db meter on it but it's very quiet and I'm sure would pass any meet requirements.
The cans themselves have an excellent reputation for durability, just that the packing tends to blow out....the stainless steel wool should fix that.
If I was to buy a new can, I would seriously look into the Laser Pro Duro, but for the price I'm very happy with my Supertrapp incarnation.
The IDS series has a bad rep with the KLR riders, they tend to fall apart with hard use offroad. The 4" series was more solid.
Oh one note about the 4", the exhaust exits radially and the pipe is fairly short in length. This translates into your needing some whipped up heat shielding for running soft bags and for extended riding to keep the heat from melting down the back fender. No small issue for pulling long continuous hauls, almost forgot to mention that.
[This message has been edited by Kingsqueak (edited 24 December 2003).]