I received a reply from V&H re my request on if they had dyno figures on their Monsters fitted to an F6B or standard GW. (Went all out in their response as well. Below in its entirety!)
Either way, I'd still look at purchasing a set.
Best wishes all.
Ken

"Unfortunately we do not. There is roughly a 3-5 hp increase over stock. Thank you for your interest in our products. "

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:: Chris Neely :: Electronics Product Manager

VANCE & HINES