Thanks, RM7. I didn't realize that smaller wheels are MORE problematic, but I guess it makes sense if the interference is towards the center. My current 18x8.5 ET35 fits just fine, I'll have to see how much clearance there is.
Getyourwheels seemed to think 18x9 would also fit - it's only 1/2 an inch wider, so about 6mm wider towards the strut - so at worst, a 5mm spacer should probably make it work, assuming I have more than 2mm clearance now?.
According to this:
JDM - KIA - Kia Stinger - Page 1 - Road Racing Wheels, 18x9.5 ET38/40 fits the Stinger, but if I understand correctly, the larger the +offset, the further inwards the wheel moves? I have sent them an email, but it seems that would be the wrong way to adjust the offset for such a wide wheel?
(slight roundoffs below)
Although, I guess, offset is relative to wheel width? 8.5 = 216 mm, so halfway is 108 mm. ET35 means (if I understand correctly), 143 mm from the hub towards the inside, 73 mm outwards from the hub.
18x9 = 228.6, halfway is 114. So again, ET35 is 149 towards the inside, 79 outside. If 143 is the limit, then ET30 would be 144 / 84, or basically ok? Or 5mm spacer, basically same result?
18x9.5 = 241.3, halfway is 120.5. ET35 is 155.5 towards the inside, 85.5 outside. ET25 would be 145.5, 95.5? Or ET30 + 5mm spacer? ET40, based on on roadracewheels would be insane, with 160.5 inside, 80 outside. I guess it would be somewhat "flush", but I don't care about flush, I want it to fit / work and not be pushing the limits too far.
I will have to look at my own setup - 18x8.5 ET35 in the front has no issues, but it limits the larger tire sizes if I decide in the future that I want to go staggered again. 18x9 lets me run 245 square or 245/275, which is a "matching" staggered setup. Not thinking of running 255s in the front at this point - neither of the tires I'm interested in have a 255 in 18 or 19. Just 245 and 275.