Which wheels are these? 18 vs 20 Ride Quality
 

https://cimg9.ibsrv.net/gimg/www.jag...c767c087ce.jpg

Looking at R models - was interested to see this wheel type - I believe this is not the 20" Gyrodyne, correct? Doesn't look the same type as many of the other R stock's I've seen.. Does anyone know what wheel (& size 18 19 or 20) this is called? Further, it got me thinking.. if the owner of this one swapped out the wheel, is there a particularly discernible ride quality difference? Or is the difference not particularly great between having 18s or 20s

That's the Turbine wheel which comes in 20" only.
All R's left the factory with 20" wheels.
And it's definitely the 380 mm front brakes over which 18" wheels will not fit, so if you hanker for an R it's 19" wheels at a minimum. And even then you will struggle to find any R with 19" wheels, it would have to have been a special order or a previous owner swapped out 20s for 19s.
According to most reports there is a detectable difference in ride quality (less harsh) between 18" and 20", but it's not huge and most find the ride on 20's acceptable.
The difference between 19s and 20s would be even less.
Just go for a test drive in an R, set the suspension to "normal" not "dynamic", and see how you feel about the ride quality.

20” Turbine. The tire size indicates the size wheel, but I can’t quite read it in your photo to confirm.

I can read the tyre size on my screen, it's a 255/35/ZR20 so def 20" wheels.

I drove around on 18s for a while, and then swapped for custom Avant Garde 20s - really didn't notice much of difference...

I'm on vorsteiner 21's with Michelin and the ride isn't noticeably harder than it was with OEM 19's and P-zeros. There is a tiny difference on bad roads, that's all.

Science tells us that given the same overall diameter, that increasing the wheel size (decreasing the sidewall height) will transmit more road noise to the chassis as you have less sidewall to absorb it.

However, it's not quite that straight forward. Most people also switch tire make and model when they also switch wheels. Tire construction also is a determining factor in ride quality. A tire with stiff sidewalls will transmit more road noise to the chassis than one with a softer sidewall.

Lastly, some people are more sensitive than others to road noise. When I use the term road noise, I'm not necessarily referring to audible sound, but any and all bumps and vibrations that are transmitted through the tire into the car. Some people won't notice an increase in road noise while others will notice any slight increase.

I suppose there are really too many variables! No subsitute to just driving one and seeing how it is. But in any case, I would take bumps and a raucous stock sound over fewer bumps and a quiet eco sound or whisper.!