It isn't coded correctly and does underwhelming damage even with pretty much the best gear in the game. I don't know if it's related to modifiers or ftp values but compared to retail it's a shell of what it should be. Unrelated but Mighty Strikes crits don't proc on WS like they are supposed to.
For anyone considering taking up Warrior keep in mind that Axe/Ridill will dominate anything outside of a relic GA here.
The modifiers are actually correct, based on what you said in regards to Mighty Strikes not processing on the weaponskill, it would appear the issue is the weaponskill is missing the critical hit element entirely. Increased Ftp would only increase critical hit rate for this weaponskill in particular, but if it's unable to critical, that could explain a big reason it's so underwhelming.
There is more to it than that, though. A simplified version of retail's damage formula is being used in certain parts of the equations here. In particular the calculations pertaining to Pdif. Cratio appeared to be closely functional but I never did in depth tests to see the extent of it (and it being functional with a simplified formula hurts players more than it did on retail against enemies T and above.) Best bet would be to not really reference retail where this is concerned, and as you said, work off with what you can for now.
I feel like a lot of the major weapon skills on content above level 75 (whether that be merit mobs, HNMs, dynamis, etc) do not do as much damage as they should. A BiS relic SAM shouldn't be doing barely 200 damage on fafnir with a sneak attack tachi gekko, for example.
Doesn't help when Tachi: Gekko, and the WS like it, are missing their innate +100% attack boost.