I haven't made any tire experiments with mine, pappy97, so I can't address that . I have the 19"s that came on my car.
I do agree with you that the ride quality is not up to what you expect from this class of car--much too harsh. It's not nearly as good as my old Town Car, for instance.
Just in case you don't know, the car was made available with air suspension most everywhere it was sold except the US and Canada. I figure, myself, that's the reason for the poor ride. The suspension they cobbled up for this market is just not quite up to the job, and I'm not at all confident different tires would change it.
However, that's just my opinion. If you decide to make the swap, let us know how it goes.
2016 V8 VIP+, black with white interior