Hey, I've done about 5k miles with the 17" run flat tyres and I can say they are alright. Not great though. Because for the first couple thousand miles I was wondering why the hell the steering at high speeds became so imprecise and wobbly. Like when doing long smooth bends on A roads, I had to basically "catch" the car as if it was constantly trying to steer out of the lane. Then I switched the steering from "comfort" to that other mode ("sport"?) and almost immediately felt a relief. It either got better, or I just learned to live with it.
As for the ride, 17" run flats don't feel any better than 19" regular tyres on bumpy roads. I can't tell for sure since it's been a long time, but it's almost exactly the same.
In the end, the only thing I gained with this replacement is the peace of mind. Perhaps it is worth it. Although, almost no one seems to be repairing even absolutely innocent punctures in run flat tyres in this country 🥶
If I were doing the replacement again, I'd probably go for 18" or even keep the original 19" wheels and buy runflats anyway.