Not a valid "test". The cam was the limiting factor, so while it picked up some power changing to the HP manifolds, the test doesn't explore how the motor would respond with a real cam. Not saying the HP manifolds are worth more than 5 hp, only that the referenced test didn't do squat to give real info. Update the motor to make the exhaust the bottleneck and then change them and then (maybe) we would have some real info.
My opinion on the manifolds is as follows, and admittedly is worth zilch. Unless you are in some kind of competition and need every tenth, who cares. Put them on and enjoy whatever the motor makes. Reminds me of the guy with a "500 horsepower" motor that goes from the trailer to the grass and back again, it's just chest thumping and posturing. So what if the motor makes 500 with headers and only 450 with manifolds as long as the owner enjoys driving it. There will always be someone faster on the street anyway, and (far as I can tell) the OP isn't looking to tangle with the Street Outlaw guys.