If you want to keep it "factory" get the proper Mini kit - pick up one off Ebay

Bit of an update for anyone who might stumble across this thread -

The initial positive reaction was not a case of initial euphoria, the mini truly is transformed into a very enjoyable liitle pocket rocket, im glad i didn't compromise to stay with 17'sw.

An added bonus has been a c.10% improvement in fuel economy which i really didnt expect - around town we now get c.24mpg! It's no diesel but thats not bad at all

All that remains is to get the rims painted white and to put some spots on - cheers everone, one happy mini owner!

Has anyone swapped to non run-flat but stayed with 17s?
The reason I ask is because I have both 15" and 17" wheels for my MX5 and to be honest, when they have the same tyres it's very hard to feel any difference between them and that's on a very modified car that has amazing feedback. The 15s feel a little more lively but busier whereas the 17s feel slightly dulled but more refined but the difference is tiny and I don't think anyone who wasn't driving it all the time would feel the difference.
On that basis I'd suggest that the biggest improvement will be from changing to non run-flats and that the change from 17s to 16s is likely to be a very minor difference.

Of course it's also quite possible that as a Mini is very different from an MX5 they do react differently to different wheel sizes.