The salesman was adamant that the XJ ride was the same on 18, 19 or 20" wheels. However, my own experience with all other cars would indicate that the low speed ride quality is directly proportional the rim to road distance and inversely proportional to how good they look.
So, I suspect so, but can't confirm.
I was told the same, and like you, my experiences with other makes differed. However, having driven some XF's with different size wheels, the salesmen are kind of right and I could not really feel a radical difference between the the diferent wheel sizes, my passengers even less so. I've not driven a new XJ yet so can't comment on that but I'm wondering if its a similar situation.
I owned, up until a week ago, a W211 E Class riding on airmatic with the optional 17" wheels. I have now just bought an XF Portfolio with 20" wheels and 255/35 tyres as standard, and the ride quality considering the wheel size is very good indeed in my view. I have driven and hated the comfort of BMW's with 18's and sports suspension and Audis with the same and there is just no comparison in ride comfort. Another regular driver of my old W211 reckons that the Jag rides as well as the Merc did on Sports 2 setting. My view is that its not *quite* that soft as you seem to have a bit more road feel, but its not that far off it.
My guess is that its the spring / damper settings that Jaguar use that seems to cater for all the stock wheel sizes quite well, as AFAIK a sports suspension (i.e. BMW M Sport, Audi S line) is not offered on the Jags other than a dynamic setup which uses a CATS type suspension, which is a different thing. Of course, I could be talking b***ocks here
The whole ride comfort thing is pretty subjective; whats comfortable to one person may be wallowy to another and so on, but from my personal experience I was really pleasantly surprised at the comfort on the 20's. Yes, ultimately the laws of physics say that the same car on the same suspension with taller tyres and smaller wheels will ride softer, but in this case I'm not sure the difference is as big as what you would normally expect it to be.
Anyway, hope I have not drifted off topic too much, just thought i would comment