Okay, let's just pause for a moment and grant the idea that Vergil's Judgment Cut is FTL. What does that mean toward how it'd function in the fight itself?
Well we already went over that, actually. Because at most we've seen the FTL is part of Yamato's iaido, and not movement speed or anything else, so it makes his iaido slashes really damn fast, but it may not apply to anything else, similarly how other Devil Arms offer bonus abilities only when they're equipped, like Air Hike or whatnot.