We tested this last year too. Went from center dump Truck manifolds tot the fancy HP manifolds and gained about the same you did, nothing.
And the small gian with the headers is about right for a stockish, low performance engine thats no flowing much air thru it. When all is said and done and you have the most power out of this combo, it would be interesting to see what happens when you put the manifolds back on. Wonder if it would only be a 22hp drop????
