I always thought this was due to the fact that the shackle kits had the *** ends so jacked up in the air that since the car was always going downhill they got better MPG and more speed.
Horsepower vs. horsepower, bone stock the headers are going to add to torque and horsepower? Noticeably? Maybe, maybe not.
There are advantages to manifolds over headers, though, especially "back in the day," (given some of the brands named here: Blackjack, et al). Ground clearance for one. Manifolds didn't scrape the street with every pothole, bashing the collectors and negating the reason you put headers on to begin with. For those of us in northern climes manifolds didn't rust out in a winter or two.