I feel you'll have to go beyond the limits imposed by the form of a sofa bed to get comfort. Sleep On Latex has 3" latex toppers in different firmness densities. With a cover and either making the bed with the right depth sheet after you fold it out and add the topper, or using the topper in a duvet under a top sheet and blanket you can get a good sleep. Store the topper in a closet. You'll need to put the effort in the get what you want.

American Leather sleeper sofas are fantastic. In my experience any sleeper sofa, no matter how great the construction, that involves a mattress getting folded in half to close the sofa will become incredibly uncomfortable. American Leather (which makes non-leather also) has a unique mattress pull out design.