Actually, if we're talking about the current situation specifically, even a really hastily thrown-together AH/solofarmed gear NIN should be able to exceed the type of numbers Sange is currently realistically able to put out on a regular basis using Manji Shuriken. Even if they didn't it still really isn't worth it to waste space on a RA set:
I'll just throw together a set quickly even a really casual player could have- note that most players would probably wish they could choke you through their modem if they saw you wearing this stuff fighting anything serious. I don't think the items I put in this set are unreasonable at all:
http://www.ffxiah.com/members/item-sets?id=261001
45 base damage
387 base delay
22% haste
Dual Wield +10
45*2=90
18% DA
So 2.18 attacks per round.
2.18*90=196.2
I'll be lazy and just use 22+15% for haste assuming you have white magic on you.
45% Total DW.
387*0.63*0.55=134.0955
123.0955/59=2.27s per round
If we take the same assumptions Kaerin made for throwing one shuriken earlier, you deal 396.72 damage in the time to throw the shuriken. Let's also say you get 20%TP, and let's say you do 800 damage on a WS (Jin or Shun I guess). That's 160 damage for a total of 556.72 damage, which is almost as much as the damage done by a relic shuriken. Considering how commonplace the gear I've chosen is, this really isn't an acceptable state.
Though, really, how would they make a relic shuriken anyway? I guess it could be dispensed like some of the RNG arrows/bolts, but then they would need to make it dispense at least a stack a day or something. But if they actually made it like that then Kaerin's suggestion of like, 450 DMG IS reasonable because I can swing a Kikoku or Kannagi all day, and I can't say the same for some relic shuriken. Hell, I probably wouldn't bother (if we're talking about it being about as difficult to get as any other relic) even if it had 500 base damage.
So yeah, I'm putting my hopes in the ranged attack delay adjustment they plan on doing. I expect to be disappointed, honestly.