I don't think Dead Aim being applied to Jishnu's Radiance would be over powered in any way. Jishnu is actually somewhat lackluster outside of Abyssea. 60% DEX mod ranged WS with the ability to crit might seem impressive, but with 3.750 total fTP, 3.945 if you're using Gorget/Obi, it's really not that great. Even if all three hits are critical (and even land, for that matter,) you're looking at 4.6875 (or 4.9316) psuedo fTP. So you're already behind Sidewinder fTP (and 200TP+ Refulgent Arrow alsp.) You truly need critical attack bonus for Jishnu's to have an edge over those other weaponskills other than just the aftermath and at this point there aren't many options for Ranger critical damage bonus.