I have been running the Indy's in the stock size since last July. I have about 8000 miles on them now. This is my daily driver tire, rain or shine (no snow). I use them in temps from about 30 degrees all the way up to over 100. Most of the time in the fall, winter, and spring it is about 43 degrees and raining in PDX, so I have extensive experience in wet.
In every situation they have been exceptional. I am realistic about these tires. They are not track or autocross tires, nor are they designed to be driven at 10/10 all the time. These are high-performance summer tires that excel as a daily tire with more significantly more grip and responsiveness than an all-season. I am of the mindset that you should use the appropriate tool for the job.
There are some people who complain that the Indy's don't perform exceptionally well on the track or at the extreme limit. I think some of what they say is true. If you are looking for a track tire or a tire where you can thrash it in the mountains, you need to be looking at an RE-71R or Star Specs or some other extreme summer tire. This is not that and it is not trying to be that.
The advantage of this tire is that you can run it reliably in cold, wet weather and it will perform well while still providing the performance you need for spirited driving. I had a big argument about the Indy's on FB the other day with people who say you should just go straight to RE-71R's or Star Specs and just run those all the time and that the Indy's just aren't worth the time. I quickly discovered that the people saying that lived in AZ and TX where it's warm and dry most of the time. I live in the PNW where that is certainly not the case. I simply cannot reliably run something that aggressive all-year. Extreme Tires are MUCH more sensitive to cold temps and their glass transition temps than even the Indy's. I don't need to drive the car in the snow, as I have another vehicle that does that, so snow tires wouldn't make any sense. I guess I could run all-seasons in the fall, winter, and spring - though the FiST really deserves summer tires and good all-seasons are quite expensive.
The other reason I think the Indy's are a great daily tire is the tread life. I should say I have not personally run Extreme Tires and cannot personally comment on tread life, though I think you will be hard-pressed to get the 25,000 or so miles that you will get out of the Indy's. Even though I am not one to cheap out on tires, if I don't have to drop $600-$800 on tires every year-and-a-half, I am happy. The Indy's have been wearing like iron, especially for a summer tire.
To sum up, the Indy's are the best daily driver tire for the Fiesta ST in moderate climates. If you live somewhere where you get a bunch of snow and ice or well-below freezing temps, you should be running the Blizzaks, which they make in our stock size. The Indy's combination of price, performance, treadlife, and availability make it the obvious choice for those of us who put miles on our cars but want performance at the same time. At $80 a tire, it's almost a no-brainer.