6UL wheel fitment
90-97 Miata 15x8+40 225/45/15 Hankook RS2 fits front and rear, no rubbing. 245/40/15 fits rear, requires rolled fenders, front requires rolling and rubs slightly at full steering lock. +20 requires 195/50 or smaller, rolled fenders and extreme camber front and rear. Clears Wilwood BBK with two piece front rotors. Fits 01+ sport brakes.
99-05 Miata 15x8+40 225/45/15 Hankook RS2 fits front and rear,no rubbing. 245/40/15 fits rear, requires rolled fenders, front requires rolling and rubs slightly at full steering lock. +20 requires 195/50 or smaller, rolled fenders and extreme camber front and rear. Clears Wilwood BBK with two piece front rotors. Fits 01+ sport brakes.
Now the situation may be different given a different brand of wheels. I don't see why you couldn't get a 15x8 +30 to fit but it all depends on the particulars of your setup. Remember that tire size is a nominal rating and not all 225/45R15 tires have the same physical dimensions. The actual section width may vary by 10mm or more between two different make/model tires of the same size.