Just taken my 19" rims off my car so I can leave them with my garage this week and get some new rubber on. I've put on my 17" winter tyres in the meantime (so my better half doesn't have to spend any time waiting at the garage and faffing with baby seats etc.).
Tyre Pressures 17" 2.3, 2.7 bar, 19" 2.6, 3.0 bar as per door post.
Having driven the car I can report:
1. Ride
On 19" RFTs, cats eyes feel like you are running over mini bricks. 18"s weren't much softer.
On the 17"s non-RFTs, the same cats eyes are barely noticable. Its an amazing difference!
2. Handling
On 19" RFTs, the handling is superbly sharp and very planted. Similar to 18"s, but even better.
On 17" non-RFTs, the car wallows much more, and feels much softer at the back particularly - it feels like it rolls more, which must be the give in the tyre.
Now the difference in tyre pressure will have some impact, but even so I was surprised how different they were. I much confess if my coming 19" non-RFTs are anywere near as wallowy as the 17"s then I won't like it! I do hope they won't be.
19"s
17"s
(looking at the bottom picture, you'd have to say the E46 was a much better proportioned design)