actually there is very little to no gain from the $400+ dollar headers to the $100 dollar headers.
on the miata boards a dude bone stock with a racing beat header ($400), test pipe and 2.25in exhuast made 105whp on a bone stock miata. same dude bought and tested the raceland header ($100) and it made a 104whp. that 1 single HP is not worth the extra $300 dollars extra.
and n/a miatas arent worth it (unless you go for the fully built high comp pistons, alm. rods, strocked out, proper clutch and ITBs with the best flowing exhuast ever.) for what your spending lol you seriously wont feel a big difference. just a little umph, thats it. 1.8s are kinda gay cus they carry the EGR stuff and you gotta connect it or remove it causin' you to run a bit richer and you gotta block it off properly.