My next question: Is it a flat expected loss (eg. ~30% across the board), and if HU/RA can mitigate it at a better rate (thanks to +hit and photon affinity) than a FO can?
Yeah I don't feel like doing math right now since I just got out of work and I'm a bit tired....so here's a generalized example:
IGNORING a Weapons %s (Native/A. Native/Machine/Dark/Hit) since those are generated randomly and a IGNORING a Weapon's Special Attack (Since Charge/Demons/Hell/Beserk can make a big difference in DPS).
Lets pretend that a Lv.200 your total ATP is 10 so if you were to equip the following Weapon types you get that much Damage Bonus:
Saber: 4.4 (Single Target Hit Weapon)
Dagger: 2.4 (Single Target Multi Hits Weapon)
Sword: 4 (Multi Targets Hit Weapon)
So as you can see Multi Target Hit Weapons don't take such a damage lost but they only hit 3 times like Single Target Hit Weapons but Single Target Multi Hits Weapon can hit up to a minimum of 6 times so it's not a huge damage loss either depending how many hits landed.
As for ATA? I'm not getting into that but your basically pinning your (After Equiping a Weapon) total ATA - your targets EVP + your Weapon's Hit% (if any) ÷ by your luck (because even if you have a 99% chance of hitting an enemy you can still miss).
This is a grand generalization but it gives you an idea and like I said a few Rare Weapons don't follow/break this generalization.
Not to mention I ignored the difference in stats between classes, Attack Speed animations being increased by Units (Heavenly/Battle and V101), and the Attack Pattern a player is using (Normal>Hard>Hard or Hard>Hard>Hard etc) really it's a whole lot to calculate.
Long story short Forces get the shaft.