Reefburnaby originally used a 4X32 (about 100W net) ballast on a 48", 32W T8 bulb, but then determined 2X OD is a better solution like GROSSR pointed out.
IMO, the most you should be overdriving an 18", 15W bulb is with about 30W. That would be 2X. You could always try 45 or 60W (3X + 4X) but the extra heat and loss of efficiency may be a drawback.
Be careful, but more importantly - be smart.