You can mount high levels (on either side.) I bought a second hand set of Termi slip ons from someone on the forum for 200 quid. They were on an S4R and fit just as well on my S2R 800. The catalytic converter for the S4R and S2R is the same part number so in theory slip ons for an S4R should fit an S2R (someone correct me if I'm wrong here). There are loads of other options apart from Termis. Saw a Zard full system on someone's S2R the other day and they've given me something to save for
. Spareshack also has a some good options (Sil do a set of high levels for the S2R)