Just saying that pointing to that test and saying the HP manifolds or headers are worth only ___ hp is flawed. Put something better than the broomstick cam it came with in and make the exhaust the restriction and (I think) the differences will be greater. Which means the magazine test is useless when applied to something other than a 300hp crate motor beyond the generalizations that tube is better than cast iron.
True but it still helpful, it's not like all of a sudden there's gonna be 20-30 hp between manifolds, not to many are gonna try to run manifolds on a 400 hp plus engine, a 350 hp test engine would of been better.
To me the test shows no point spending money on 340 manifolds for power gains any 360/5.9l are good enough. And if you already got a good dual exhaust with 318 manifolds not really worth the swap either.