I've seen films where Corsairs would haul huge bombs and napalm tanks and make stikes at forward positions, they didn't retract their landing gear! With no fighter opposition, you're basically flying a giant dump truck!
I've seen rattings for the F4U-4 at 4,000lbs. The P-38L also was rated at 4,000lbs but in war it was sent out on occasion with 5,600+lbs, it would not be unreasonable with a long runway available for a Corsair to be overloaded occasionaly. A lot would depend on hard points to carry the load with.
Here is a site with an interesting article on the F4U-4 (note the P-38 references) http://home.att.net/~historyzone/F4U-4.html The home page has some excellent articles on the P-38, Republic Aviation and the development of the P-47.