Hello - I am driving my second XK8 2005 convertible (this one is a Carbon fiber edition) and I have to say both cars are really well made, very smooth and quiet riding. Surprisingly quiet at higher speeds even wih the convertible top.
I cannot speak to the ride in a coupe but I imagine it would be somewhat quieter, but the convertible is quieter than you would expect - drive one and you will see.
Not sure about the stiffness vs an R version - probabaly very similar - have not heard much about any difference here.
Beautiful, well built and quiet touring cars for sure.