When you turn the wheel to one extreme, pump pressure is maxed out. If you had access to a power steering system analyzer then that's one of the things you do to check the max pressure the pump is able to create.
The largest reason I can think comes in to play because of the nature of power steering systems. The system was never designed to be at full-lock AND wide open throttle like we see in drifting. In a daily driver, if you're at full lock then you're probably pulling in to a parking spot and obviously not at wide open throttle.
The safest thing you can do, if you want to keep power steering, is to get a larger reservoir but NOT increase the fluid level substantially. You want more room for that fluid to flow but you may need to add just a bit more fluid to prevent starvation.
The other option is to delete your power steering all together and find a manual rack or route the lines from the gear back in to each other. Do NOT run a power steering rack empty.
Doug
