Header Tube size.

Car Craft did an article a few years ago on a 700hp big block Chevy. They tried 7 different sets of headers and found that even the smallest header ( 1 3/4 ) was within 30 hp of the 2 1/8 headers. The 2 1/8 headers were dyno specific headers the had 3 inches of straight coming out of the port which is worth a bit of hp in itself. The difference between that your going to see is pretty insignificant.