I did some digging into this a couple years ago. The BHAF part number you want is AH19037, don't go with the AH19002 which looks the same but flows less than stock (it's a Marine version).
Compared to the stock air filter it's not a huge improvement (note though that the stock air box itself has considerable restriction and drilling large holes in the lower section help quite a bit).
Here's some info I found when researching this a little, CFM ratings along with measured restriction (inches H2O):
Stock 2003 air box with filter is roughly 500cfm @6"
and these filters match the Fleetguard
Donaldson B105006 590@6" 680@8"
Wix 42790 680@8"
Napa 2790 680@8"
Fleetguard AH19037 590 @6" 680 @8"