Just my opinion but for racing you want a wheel that is the same as the section width of the tire (305 ~12") for spirited street driving, the same as the tread width (305~11"). Very low profiles are more sensitive to wheel size than taller tires and with too narow a rim you need to reduce pressure to keep all of the tire on the ground (too narrow a rim will make the center crown and wear the middle of the tread quickly).

If you really need 305's then I'd suggest two sets of wheels and tires: one for street driving and a second set for "other".

ps In general you can use a 3mm spacer and maintain the hub centric loading. More and the spacer will need to maintain the lip.

pps for maximum cornering about a degree negative camber doesn't hurt.