I'm not sure how you arrived at those numbers, but I do now believe the 18x8.5+30 is an accurate size for OEM. Given that the XSE hatchback wheels are 18x8+40 with a 225/40/18 tire, it's not unreasonable to think that the GR Corolla with wider fenders and bigger brakes would run a 18x8.5+30 from factory on a 235/40/18.
This is new territory for me since I've primarily had experience with STI's and Type R's where the common 18x9.5+38 sizes are the sweet spot. I'm thinking this size of wheel would be a difficult fitment to clear calipers and suspension without at least a 5mm spacer.
Design also matters. Whereas the oem GRC wheels have straight spokes from the center to the edge of the wheels, wheels like mine have more concavity on the backside of the spokes to allow for bigger brakes
It's simple math once you look into it. For every inch wider you go for the same offset, you lose 12.7mm suspension clearance and poke out 12.7mm outside (1 inch = 25.4mm). Since it's a 38mm and not a 30mm, you add the 8mm to get 20.7mm, but round it up to 21mm closer to the suspension (higher offset, more towards the suspension) and subtract 8 from the outside, to get it poke out 4.7mm more, rounded to 5mm. Conversely If it was 22 instead you subtract the 8mm, so you get 4.7mm, rounded to 5mm, more towards the suspension, but add the 8 on the outside so it would poke 20.7mm more out, rounded to 21mm.
So when I mentioned an 18x9.5 et30 or et22 would fit better for coilover clearance, it's because you would only be 12.7mm closer to the suspension, but poke out 12.7mm more (18x9.5 et30, offset same as factory wheels) but et22 would give you 4.7mm closer to the suspension, but poke out 20.7mm more than the factory wheels.
But all I'm saying is that for 18x9.5 et38 with coilovers will be very close to the suspension. You will get a more suspension clearance on factory style struts.
Big brake caliper clearance is a totally different conversation.
