Those test did not change the carb setting IIRC.
FAST cars are a totally different animal. Same as people that think Stock Eliminator cars are "stock" :)
If 340 manifolds were the same as headers, stock or super stock cars wouldn't run headers. Those guys are after any marginal gain.
One 340 I have ran like a dang 3 legged crippled dog with 68-70 free flowing (yeah that's it) exh manifolds. Only change was a set of header and it ran totally different, much better.