Yes, moving up to a D rating will be an increase in reliability. And not just at higher speeds.
I had a C tire get a rock puncture on a gravel forest service road at 2-3 MPH! I heard it when it happened as the widows were down. What a pain. Almost at the campsite, and had to change the tire on the road.
While I cannot say for certain that a D tire would have fared better, with it's more robust construction, it probably would have. I was done with Cs after that, and have never had that happen again.
As for the rim PSI. I simply ran the tires at the max the rim was rated for. It worked well, and gave me some of the benefits of the higher rated tire.
As for the ride being rougher... It makes sense that it should be. But it is not a discernable difference in my experience. That TT I had the rock puncture on started life on Cs and when I sold it had Es.(80PSI on new rims) Not a difference in ride that I could tell.
What will improve the ride greatly if that is a concern is installing one of the shock absorbing equalizers on the market, Trail air Equaflex etc. Those flat out work and make a huge difference.
