I doubt bigger wheels shows up on 99.999% of small car buyers wish lists. It is MPG, then price, then interior room, then cargo room, and way down at the end is performance and image. The only real questions I get concerning the Fiesta (which started back in early 2009 with the EU car) has been all about MPG, price, and interior room. Going with a shorter roof height and larger wheel openings reduces the interior space, which is a prime complaint of small cars. Comparing the height of the Fiesta to a C7 Corvette is a little silly. One is a 2 person sports car and the other is a 5 passenger econobox. The Fiesta needs to be compared to cars in its class, which is the Fit, Sonic, and Versa. They all tend to have high roof lines and smaller wheels. The Mini is more of a niche' car, and not a high volume seller. People buy those for image more than function. The Fiesta ST buyer isn't the typical B-segment person, but it would be too expensive to develop a stand-alone B-segment chassis just for the ST. The Puma was the closest thing to that concept, and it did not last too long. (Sad since it was full of awesomesauce)
I am 100% happy running 17" wheels and would not want to go any bigger. I will be going to a 215/40 tire size to get a little extra sidewall for rim protection and wider foot print. The visual problem with going with bigger wheels is that they make the already little looking brakes look even smaller.