Don't run at 1/2 ohm. I've been using AP1500's for a year now and they couldn't handle 1/4 ohm guaranteed. Even during a burp.
As others have mentioned, the key to getting loud:
- test for optimal box size
- test for optimal box placement
- test for optimal tuning
- seal and stiffen everything (box, rear quarterpanel vents, A/C vents, edges of plastic panels, etc)
- test various positions for seats, headrests, visors, doors, windows
Getting to know your equipment is the best way to become a world-class competitor. Spend the time and test systematically.