Well, what a revelation in comfort! We are driving down ro Croatia so stopped off along the way and fitted the conversion kit in the car park. Gregor was very helpful and set up a table for us so we had everything layed out which was useful. We also had a lie down on his demo bed, so we tried the version with different spring stiffnesses as well as the ‘universal’ one with same stiffnesses. We fitted the version with ’universal’ springs and 4 flat longitudinal strips. We also tried (but did not buy) the squergo mattress which is softer and a little thicker than the standard VW one. The work took 3.5 hours including lunch and a tea break of course.
The reason we did the conversion was to solve the roll together issue which had become a nuisance. The new bed is on the firm side, very comfortable with no roll together. Also it does indeed feel wider.
A great improvement. Thanks to the OP, bvddobb, for your detailed post