I looked in both Factory Service Manuals for the WRX (2.0) and STi 2.5 and according to the factory the capacities are the same (~4.2qt). That being said, and having personally spent a lot of time looking over the lubrication systems from both motors, I think the STi pan is a better pan especially for anyone considering track conditions. I've not heard of anyone running 6 quarts. The most I've heard of anyone running was ~5qt at the begining of a track session. Anything over that and I'd think you'd risk sloshing into the crank, under acceleration or deceleration, or backing up the return from the head, under lateral load. Let me just say that this is simply my "6 quart theory" not anything I've ever actually tried.
While the WRX pan is wider at the bottom it is also not as deep. The STi pan captures the oil around the pickup and gives it less room to slosh around and it's baffling gives a more direct (and directed) path from oil return (from the heads) to the bottom of the pan vs. the WRX pans double tier baffle where oil can cascade down 2 levels before reaching the bottom of the pan. I've not installed every header out there, but from what I've read on the forums the WRX pan is also a lot more restrictive for header choices than the STi version, not just limited to EL or twin scroll headers interference, but others as well.