Obviously, different combos will yield different results.
I know someone with a pretty good running sb mopar in FAST that tested stock manifolds vs extrude honed ones back to back on the chassis dyno.
With his combo, it was basically worth nothing. I've found this myself. If the manifold is at its limit power wise, most anything won't help it. A bunch of circle track guys were doing it back in the mid 90's. Never found anything on the dyno or track with those particular manifolds, which were some GM junk.  That's the same I've seen. I used to build some roundy round engines for a few locals and they've tried some things w/ exhaust manifolds...none of them proved to be worth it. Got to where all I'd do was gasket match the manifold to the head and then open it up at the collector flange.
CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax
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