Yes the STi pan would not hold 6 qts. Even with a WRX pan I think that's way too much. As I said before, a couple of my customers who track regularly say 5, to a touch over 5qts, is the max. Personally I trust thier word because if there were any way to get more in there without having a negative effect they'd be doing it!
nottofast, according to the factory service manual both WRX and STi require 4.2 US Quarts. I checked the owners manual for my 2005 STi and it concurs with that, 4.2 US quarts.
Something to consider here, and maybe this is splitting hairs, but the total capacity of the EJ motors is just under 5 US quarts. So when you do an oil change there is still ~3/4 US quarts of oil in the motor. 1/2 quarts worth on the heads and oil system passages and about 1/4 quart that doesn't come out of the oil pan because the drain is not at the bottom.