What is your wheel size and do you have Run Flats? Swapping to non run flats should make a huge difference in the ride.
I have Cooper D Convertible with 17" wheels non run flats. The car came with Bridgestone Potenza's and the ride was hard but that is what I loved about the car. I had to replace two tyres and I went for Hankook Ventus but as soon as I drove the car on the motorway it felt terrible, very different and when I executed a swift lane change it felt like it wanted to wander all over the place.
Turned out it was because the Bridgestones are really hard tyres compared to the Hankooks - when I went in to get the other two replaced the tyre man got two down for me to press on the sidewalls, the Hankook had some give in it, the Bridgestone didn;t budge/ I had another two Hankooks on and now the car is fine, they said it was down to the mix of tyres and they were right.
Had I known that though I would have gone for Bridgestones again as there was not that much money in it between the two because I'm a bit weird and I kind of like that bone jarring ride, so if you do change, make sure you do a bit of research on the tyres you are replacing with perhaps to go foe ones with a bit of give in them.
As for economy, I have read that the bigger the wheel the worse the economy is, something to do with needing more power to get the car rolling on those big wheels initially. I guess it depends on the sort of motoring that you are going as if you are on a motorway bigger wheels would give better economy than smaller ones as they are not having to turn as quick. To be honest though, if you have a diesel then thhe change in the overall economy is hardly going to make any different and I would prefer to see my 17" wheels on rather than some 15" ones.