I have added a 0.5β soft foam layer along with soft leatheritte on my seats. But on fabric seats not on leather. Same can be done with leather too.
But the 2 important things are
1. Selection of good quality soft leatheritte
2. Not too tight covers
Cheap leatheritte will be hard and also if the covers are too tight which most poshish guys do, it becomes hard as it looses its elasticity after being stretched so tight.