It's worth full timing in Abyssea because 75%+ of your attacks should be critical hits. Comes out to a bit under 5% increase in overall damage. Raider's boomerang will increase your dot by a bit over 3%. Also, Raider's boomerang does next to nothing for WSs while the crit ammo will do a lot for evis.
edit: Alright alright, this post was a bit hastey. I'd have to sit down and do the math for exact numbers.
edit: Errr....I scribbled some numbers down. Didn't really put much thought into it before, maybe I'll reconsider using the crit ammo full time, or at all. Moar number scribbling is necessary.
edit yet again!: I was one of the first people to get their hands on the tathlum on my server. I don't have the numbers anymore, but when testing it, both my max and min increase was about 5%-ish.