Of course, there's a displacement advantage. We're talking about Haaarwllaayy-Davidson, they couldn't race their way out of a wet paper sack without all the rules put in their favor.
I think it's 2600cc for the Hardley-Ablesons and something like 1400cc for everyone else..
Like I said we're not talking a small difference, FWIW Ducati does just fine with no advantage at all..