When I last checked (around 2-3 years ago?), the Porsche GT3 Carrera/Cup cars had the Michelin N2# 27/65/18 slick fitted to a custom sized 18x10.25" (286mm wide) wheel.
The section width of one of these brand new Michelin slicks fitted to this custom wheel was 290mm and the OD was 645mm.
At that time, based on the Michelin's advertised section width of 298mm on a measured 11" wheel, the section width should have been 290.5mm on the 10.25" wide wheel (i.e. 5mm less section width for each 0.5" reduction in wheel width), which is more or less exactly what it was.
Note: the Michelin-published 298mm section width was from 2015. The latest Michelin catalogs specify it as 295mm.
I fitted one of these used slicks to an 18x9 and 18x10 wheel to get some further section widths and ODs for fitment data:
- 18x9, 266mm, 651mm OD - should have been 278mm/9" - actual is 12mm less!!!
- 18x10, 281mm, 647mm OD - should have been 288mm/10" - actual is 7mm less!!!
And calculated values based on 9" and 10" measurements:
- 18x9.5, ~274mm, ~649mm OD - should have been 283mm/9.5" - actual is ~9mm less!!!
And the custom wheel:
- 18x10.25" 290mm, 645mm OD - should have been 290.5mm/10.25" - actual is exact!
So, either, the used slick had shrunk and its section width was less than expected on narrower wheels, OR, the slick is substantially narrower when fitted to much narrower wheels relative to the 11" measured wheel.
I've seen various dedicated M3 track cars on this forum use 18x10.5 et22 with wide slicks (for which the N2# would have ~293mm section width based on 2015 data, or ~290mm based on the 2020 data) but you'd be better off taking your own measurements to confirm fitment on your car.