No one has popped on this thread in awhile so thought I'd add some more info for folks who want to go this route.
My 18x8 PIU wheels arrive this week. $493 USD including shipping [from Canada to the US] which is $100 less than you'll pay for 1 wheel in the US. I got mine from
https://www.rivercityfordparts.ca/
In terms of tire sizes for these wheels, 265/60/18 or 255/65/18 will not only work, our test drivers couldn't discern anything noticeably different between the two sizes when ran side by side on the road, dry, or wet track [running '22 ST's] so choose based on price and availability over what someone tells you is the "better" size.
The stock lugs will work and no spacers are required. I did grab a second set of of lugs, but that's just me and my penchant for preparedness.
As far as the width and the "empty look"...um...the OEM 21's wrapped in 275/45's aren't wide enough and have an ugly gap as-is, so potato/po-TA-to? 265/60/18's sit nearly an inch bigger in circumference than the 275/45/21's and the width difference is < 7 millimeters. When you see them in person the fuller height makes up for the slightly narrower stance, at least optically.
Ultimately, it's a preference thing, entirely subjective, and that's cool, everyone should be able to enjoy what they enjoy, simple as that. For me, I'm simply hard on my vehicles, I drive 50-60k mi/yr and range from highways to NFR's. So PIU wheels with some MI CC2's make the most sense for me and I can live with the look. Worst-case scenario, I get tired of it I'll go with some gloss black 20" Savini BM13's with 255/55 PS4's and keep the CC2's on the PIU's in the garage for a rainy day. I think that's my favorite thing about the ST, it's virsitiliy in function and in look. It can be an absolute sleek and stylish badass [look at all unbroken has done, it's killer!], they can be absolutew sleepers, or they can go unnoticed, totally blending in with the surrounding vehicles. I'm leaning towards mine blending in at the moment.