Love this thread. Right up my alley!
I'm looking at downsizing to 19s for autox, and this thread has the right
technical information to make a determination instead of just "they fit".
Yeah, the washers is an old trick to test the needed clearances. You do not need much space between calipers and the wheel. They don't flex much. You do need to worry about outboard (fender) and inboard (shock/suspension) clearances more because of tire flex under load.
It would be great if you can get by with just a 5mm spacer... of course, "no spacer" would be ideal.
BC... do you also have the 6pots like Crank? I'd love to get some measurements of their size differences.
--kC