I dunno. The instakill one will be 10 + 30 (character Level) + 40 (90 wisdom) + 15 or 16 (Assassinate DC) = 95 or 96. That's probably better than what the Assassinate ability from rogue gives, which is 10 + Rogue Level + Int/Dex Mod + assassinate DC. Assuming the same stat bonus and gear setup, you are looking at 10 higher the DC than a rogue gets on their instakill ability. And this tree gives +3 to assassinate DC as a low tier enhancement. Rogue assassin doesn't get that.
I don't have a problem landing Assassinate today. 10 higher should be fine.