I do believe the darius buffs currently on pbe to be agree-able and sufficient
Nothing more need be done after that, for fear of over-buffing. Hopefully this is enough to bring him from te depths of WR hell to a respectable 49-51% range.
If it for whatever reason happens to be a bit of an overbuff the bleed can go from 13-30 to 10-30 to keep his level 1 in check (even though his level 1 cheese didn't really occur often in higher elos, at least not successfully).
Some darius mains had asked for AS, but I believe giving darius AS is contrary to the noble goals stated in his update during the juggernaut patch. He should have a reliance to spellcasting and mana. His purely auto trading SHOULDN'T be optimal at all.