Michael,
Arguably a better handling tyre at slight expense of ride quality. In my experience the P6000 225/55 has worn faster than the P4000 225/60, but with the sort of mileage you'll do, that's not exactly a problem.
It will make your speedo slightly over-read from it's current reading. But there's little you can do about it. All the speedo componnets are the same on late Touring-suspension cars (4-litre convertibles) as they are on Sport-suspension cars (4 litres coupes + 6 litre cars). See my post here:
http://www.jec-xjs-forum.org.uk/xjsbb2/ ... .php?t=682
I would just change them unless you like the current setup.
Paul