Across the resistor you will get just over 8.5V if your batt voltage is 13.8. If you use a max duty cycle of 85% this gives you 13.6W. That's still more than 10W. The 25W resistors will be far more comfortable, and much better equipped to get rid of the hear. Think of the wattage of your soldering iron - you can solder easily with 12W - it's a lot of heat!

Rick