We had some 11mm Cloud 9 in the dining room, but it was just too thick
and spongy. Furniture disappears in the stuff, causes horrendous
indentations and it feels just too much like walking across a matress!

Yeah, kinda know what you mean there...

11mm is to thick for a living/dining/used room, it's intended for
bedrooms and the like. 9mm would have been better or maybe as thin as
7mm but it does depend on the carpet as well.

Ours is only going to be for the upstairs (2 bedrooms, stairs and
landing)