Tirerack Fitment
 

I have the brembo package on my 86 and looking at tirerack's selections for 18in wheels that will clear the brakes.

If TR lists an 18x8 +45 wheel as fitting, would it make sense that the exact wheel in 18x7.5 +38 (not listed by TR for the 86 but is 5x100) should fit as well? Checking wheelitfit, the flush distance is exactly the same.

it depends on the spoke design.

Spoke design should be the same just different width/offset

Some wheels have the same design, some like RPF1 are very different depending on offset. IDK about the wheel you mentioned, but don't just assume they will be the exact same for different offset.