Just use the factory recommended air pressures.
For passenger cars, (there are also truck XL tires, but that's a different scenario), the XL tires are for very high speed use of 100++ mph. When a tire is traveling at those very high speeds, the safe load carrying capacity is reduced and the tire can blowout. At those speed that will really ruin the day of the driver. So adding air pressure to maintain a safe load carrying capacity is needed, hence the need for XL tires that can safely withstand higher tire inflation pressures.
Also when a tire is traveling at very high speeds, its shape deforms a bit to more of an oblong shape than a circular one. This can actually cause the tire to come off of the wheel and again, at that speed the results wouldn't be very...positive, shall we say. Higher air pressures can help to reduce that tire deformation.
The drawback to XL tires is that they tend to ride very hard compared to the standard, "P", rated tires.
-- B.F.Goodrich T/A Certified Tire Specialist