Having been down this road myself trust me when i say it's more about how the foam of the back rest and the foam of the base meet once in the "seat mode".. too thick and it will crumple and too thin will have a gap. The supplier of my bed suggested the wrong size foam and this delayed my upholsterer and meant tons more work, basically he had to trim it twice.
Food for thought eh,
Cheers,
Liam.
we have 100mm (4") which we got from a foam manufacturer. its 2" of one density and 2" of a harder density, the theory being sit on the softer, sleep on the harder - we like a hard bed.
unfortunately, after 9 years we cant remember nor tell which is which!
I cant see that thinner foam would be more comfortable other than it wont bend over as much at the edge of the seat?