I had 5mm slip on spacers on my back wheels to clear the coilovers. Did quite a few track days with no issues. One experienced mechanic told me it was a bad idea and a experienced racer told me that up to 5mm would be fine.

__________________
"ALLPAWS" 2014 WRX RS 40 - modding very slowly.
2002 Mitsubishi Airtrek Turbo R - Ralliart Edition (daily)
2010 Nissan GT-R - when the Missus let's me drive it :)
Crotch Rockets . . .