I last rode Xpress in 2010, it was more smooth then Kanonen (Liseberg) in 2010. The OTSR's are slightly more uncomforable though, but the ride itself is very smooth.
They should just put a mechanism in it so the OTSR's don't 'click', but should have an 'infinite' number of possible settings.
Someone on themepark.nl seemed to know that you can just replace the passenger module; you do not need to replace the entire train to upgrate to MK 1212 is what he said. I don't know his source though.
The rollercoasters in walibi holland, descending in ride comfort:
2. Xpress (SBNO)
3. Speed of Sound
4. Robin Hood
5. Rattle Snake (used to be Woks Waanzin / RoadRunner Express / Keverbaan)
6. El Condor
Rattle Snake is very uncomfortable, because their programming is really weird. It just doesn't /barely brake(s) when running through the station for the first time, and the second time it stops in under .5 seconds. Also, the friction wheels work very fast. This results in weird quirks when riding. The el condor shakes like hell. It's a fun ride, but it's a mindblender