Thats not really much of a change and either way it is still going to have to same look . But if you were going down to a 12" that would really change the look.
If you change to a 15" tire it's gonna make it look plain like a Saturn or 90's Bonneville. <-SARCASM! A 15" might even look better? It's still gonna be a wild ride.
I'd say get one and pull a tire off, roll the smaller one up to the car and see what it looks like. Personally, the car seems to be built for a fat tire, not that 15" isn't fat. But I like the extreme look you have now.
A fer the white stuff, either he lives in a boarder state to Mexico and that's left over Cochin from the last drug smuggle gone bad, or its natures AC