I would not go any bigger than 17" wheels on the Fiesta. I am really close to hitting the inner fenders with P215/40 17 tires. Going lower profile to knock the diameter down would put the wheel at more risk. Plus, 18 and 19 inch wheels add more weight, which I have found to be a performance killer on small cars. I haven't tried heavier wheels on the ST, but going to the 17" wheels on my Fiesta really drags down the performance. The more wheel weight means the more unsprung weight, more rotational mass to accelerate and slow down.
Probably lower profile tires would degrade the ride too much as well. Hence the comment about needing bigger wheel arches so you can have larger wheels while maintaining the same profile aspect for the tires.
As to wheel weight, no doubt it's important - but as I mentioned in another thread if you are willing to pay the premium I'm sure you can find some super-lightweight forged rims at 18/19" that would not weigh significantly more than the standard OEM 17s. Tires would be a bit heavier though. Of course other aspects of the car's design would also need to be altered to go with a bigger wheel package, such as suspension setup.
I'm surprised that no auto company has realized yet what it could do for its sales of subcompacts in the youth market if it designed its subcompacts to accommodate slightly bigger wheels (Mini is an exception here, but it is more of a premium market car). The ridiculously high side and rear heights of even subcompacts these days call out for larger wheels to maintain visual proportion. A FiST is 57" tall - that is 8" taller than a C7 Corvette for heaven's sake, and even 1" taller than a Cobalt SS. A Honda Prelude from the 90s, a compact class car, was less than 51" high.