I have a 98 S3 with S3T seat, tall bars and hardbags(so I can switch back and forth). I've never rode it two up with all the S3T stuff on it, but it's pretty comfortable riding alone.
I have a Lightning. I was thinking about buying an S3T for the same reason, to ride 2-up with my wife. The Lightning is too small for that. Bumping the thread to see if anyone else can comment.
Also, how's the M2 Cyclone for riding 2-up? It looks like it would also work.
Had a 97 S3T untill 2003,Im 170lbs,wife is 120 lbs,Almost always rode 2up,no problems/complaints at all,even after all day on this awesome bike,however it could use small backrest to keep passenger from flyin off..............LOL.