I am running 12.5 f/r on ZCP wheels. Many with ZCP wheels are running 15f/12r with no issues (assuming stock sized tires). Since ZCP wheels are 4mm closer to the fender stock, you should be able to run 18f/15r no issue with non-ZCP wheels.
Increasing rear track width relative to the front will increase understeer.
Audi S6 * Audi S3 * Porsche Cayman GTS
--Former BMW M3 owner