[rrobson]

I’ve got hairpins on my 75 at the moment simply due to refusing to buy a new set of tyres for the wheels that were fitted. It’s important to note the tyres fitted to these wheels is 225/45/18, a slightly larger profile than standard. I haven’t noticed much change in comfort, there is slightly more road noise but again difficult to notice.
On my zs I did the same as you’re proposing, fitted some 15” wheels instead of the 17’s and the ride is still firm and the road noise is still unbearable on long journeys.
I would say the biggest factor is the suspension, good suspension tuned to the tyres and requirements of the ride will yield a good ride with any tyre and wheel combination. However I never had an issue with the zt I had a few years ago, I thought it was perfectly compliant.