Given that most games in my experience dont reach level 20, it is kind of a moot point.
Also, stat skills for assassins are very potent at the last level, end game is mostly about auto attack damage, damage from spells and skills is secondary.
12% + attack speed
+ 36 Dmg
690 + HP
Coupled with the extra Mana and minor regen bonus, that is artifact level stat increases. However, I think the regens could be tweaked a bit, possibly +1 or +2 per level.