Pressed the reply button too early there!
Results are:
-Rear end a touch lower (I wouldn't have chosen this but it's not an issue in practice, not so much lower that it looks odd).
-Softer springing/better damping has transformed the stiff, bouncy ride at all speeds.
-Could possibly do with some more progressive bump stops as that end of travel is a bit abrupt (the car goes on some challenging roads!).
-The track is now a touch wider than that at the front. This doesn't seem to have any negative effects, contrary to my expectations.
On the road, the car is much more positive in feel. You can throw the car around with a feeling of having control over the rear, similar to a torsion bar suspended Peugeot (think 205, 306, my current Partner).
On the current cheap tyres the car came with I can provoke the rear into lift-off oversteer on damp, sharp bends and then (luckily!) catch it without much drama.
May not be quite so interesting once the Michelin Alpins go on. I went for 165/70R14 rather than 65s on the standard alloy wheels as these will offer a bit more ride comfort, more progressive breakaway and are still very close to the bog standard 155/80R13 in rolling radius. Should actually render the speedo accurate as it under-reads by about the same margin as the tyres will increase in RR).
Overall, very happy. I've used the car to do two recent 800 mile trips and it's solid and planted, with much more comfort than previously. Fitting was just a case of remove old and fit new with no messing about. The handbrake cables from the 500 fitted perfectly.
The same should be the case from a mk2 Ka. Ford really did do a great job of improving the setup.
Next job is to transfer the struts, hub carriers, anti-roll bar/drop links, steering tie rods/ends and driveshafts from a 2013 500 and brand new 500 bottom arms to the car once I get a chance. Fingers crossed this will go well....