Best way to see what kind of stance you will have is to measure and go from there. 20mm will work, but are going to be very close to the fenders and may rub. 15 are a pretty safe bet on pre-LCI and with that set up. Modifying the suspension and then correcting the camber for your coilovers may restrict that more to a 12mm.
A rule of thumb is measure from shoulder of the tire to the inner fender, take that measurement then back it off about a quarter inch to account for suspension travel.