When I took my headers off and put the monty cast iron "header" on with the aforementioned secret but illegal modification, I noticed no gain or loss compared to the header alone. I think the main improvement over stock is losing the restrictive stock cat. I suspect a 2.5" high flow cat, if needed for legality, would yield similar good results. I should say that my motor already had the Weber 38 DGES when the changes were made.

83 mitsu flat bed 2.6l weber, 88 auto tranny, 2" body lift, 4.625 LSD, 235/85-16E mudders (33 x 9.50).