I haven't had a passenger on my SMT, so I can't speak to that. But I do have a huge Givi Trekker 52 top case that you can get a passenger back rest for. It holds two full face helmets. It's a little big, but works for me as a daily commuter. It pops off in 2 seconds for around the town riding.
If you want a comfortable "super bike", I think the SMT is hard to beat as far as handling and brakes go. The 115hp leaves some on the table compared to the MTS, but I don't need the extra power.