If your fuel system is built for higher percentages of ethanol the ethanol itself won't corrode parts. Ethanol and other alcohols really really really like to absorb moisture though, so does brake fluid btw. So letting a fuel with high percentages of ethanol sit for long periods isn't recommended as it'll wick up and absorb moisture leading to corrosion. Although I'd think a modern car like a WRX/STI with its sealed fuel system would limit moisture introduction more than an older vented gas cap carburated vehicle. But yeah you really should use the ethanol fuels as soon as possible to limit the chances of corrosion. I've seen the inside of carbs that looked like they were in the ocean all because they sat for 6 months with regular gas that has the 10% ethanol added to it. Nasty stuff.

Ethanol % does vary. Some stations have higher %, some lower. The other issue is E85 isn't always 85% ethanol, sometimes the mix isn't exact, ethanol evaporates, moisture is wicked into the fuel making some of it water, etc. Interesting fact, finding 100% ethanol for sale is very hard, but 99% is available. The reason is because 100% ethanol is regulated as the kind of alcohol you drink and is taxed accordingly which for vehicle use obviously raises the price unnecessarily. But 99% ethanol doesn't qualify for the booze tax so that's what the suppliers mix up for vehicles to avoid the extra tax.