I guess the question is - what do you care about more, performance or looks? I agree that 15s don't look perfect on the car, but you'll save at least a few pounds per corner. 16s might be a good in-between, if you can find what you want.
Not sure about the OZ wheels coming in a 17x8, unfortunately Tire Rack is like the only distributor in the US for them, and they will only show you fitment details for specific cars (and if you put in Fiesta ST, they'll just show you the 17x7s...). You can ask Tire Rack though, they have a pretty good chat system that you can use to ask a Tire Rack representative. They might be able to tell you if there is a 17x8 4-108 OZ wheel.
For what it's worth, on my other car (2010 Hyundai Genesis Coupe 3.8) I went with 19" wheels, and I think they look fantastic but they are not ideal on the track. For me, I probably should have gotten some 18" wheels to save a bit of weight on the track. But the 19"s looks great on the street..... I understand your dilemma!