I know this isn't Fiesta related, but I've just fitted 17x7.5" rims and 215/45 tyres to the wifes Mazda 6 (up from 15" and 195/65.) The ride is noticably noisier- particularly over patched roads and the ride is harsher with more tramlining. It's worth it for the handling improvement around corners (a HUGE improvement) but I'm glad we didn't go for 18" rims/ tyres....
Yes, they're 20" rims with 30 series tyres!?!
Here's the full advert