The Keihans are a top shelf choice with the advatnage of being stainless.
After a certain model year (not sure) BMW OEM R100GS mufflers are SS, fit, but the bracketry needs some mods to handle the ST or G/S side cover.
Sito makes a decent aftermarket muffler, but folks I know with it have had to go ceramic coatings or other steps to slow rusting.
I have a Staintune R100GS muffler on my ST that i lucked out and bought on Ebay from the USA importer as a blemished product for half the MSRP. The blemish was a mark where a bolt had been inserted into one of the mounts. I had to fab up a couple of brackets to secure the left side cover since the muffler didn't have them. Also discovered without the optional $32 baffle insert it lost gobs of midrange and top end torque and sounded like a Harley with cut-off mufflers at expressway speeds.
But adding the baffle and the bike has never been happier - sounds great without being loud, too.
During the initial fitting. Since then I have made some brackets and installed the black heat shield. Except for the extra long stinger at the end, it looks like it came on the bike. The extra long stinger keeps hot gases away from ANY luggage you might fit to the bike. I've thought about shortening it to more closely match the original muffler length, but the performance up in the mountains tells me via the ass-dyno and the ears that the motor is very happy with the resonant length of this pipe.
As always YMMV.
At the time I bought this pipe, CB & T in CA was the USA importer for Staintune and was great to deal with. You may want to scour Ebay for any offereings from them, or call them directly and ask about clearance or blems items. It never hurts to ask.