For road riding I don't find anything more than 40mm tires run at moderate pressure is needed to be comfortable.

I shot this video on the roughest "road" [actually a long wooden bridge] that I ride regularly. I'm on 16" wheels with 40mm tires [that were harder than they need to be] - no other suspension. I actually faster on that bridge than lots of other big wheeled bikes because my tires do a better job of soaking up the bumps than their bigger, but thinner/harder tires do.