I have heard of it done, buthaven't done it myself. I have a spare set of MP3 rims in my garage that I was going to test fit to Lotsy's Fiesta but we ran out of time modding her car.
I'll try it sometime if no-one can give you a definitive answer, if Lotsy's over anytime soon.
I think Ford's problem with the setup was that the tyres need to be a 35 or 40 profile on 17 inch rubber - if you're running 40 profile on your Focus, that's pretty thin and I'd suggest your speedo is out, because 45 is the factory approved profile for 17 inch rims on a Focus.
Tim
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