Your asking all the right questions today chap.....
Now I have first hand experience of this very upgrade and your not going to like it ….well you may but I doubt it.
Yes you can use the H&R springs with your s-line dampers, but it's not really ideal.
Yes its better than the stock springs but only marginally, ride height is good and look great but the lower spring don't really suit the dampers that well, maybe because they are not designed for the shorter springs and it does effect their performance, I mean they work but its a bit of a waste of really good springs on the s-line dampers.
if your going to change springs then you really need to do the dampers aswell , I have H&R 45mm springs and Bilstein B8 dampers and can honeslty say it transformed the car, its the setup that should have been on the car from the factory, sporty and civilised , great kit.