Hmm the passive effect is clearly playable at two mana, and almost certainly playable at three mana with more flexible color requirements as well.
Being a Planeswalker is a boon in some matchups (the control mirror) and a liability in others (anything creature-based that can go around or through a few Servo tokens, e.g. Shops).
I'm not so convinced about the utility of the activated ability. What exactly are you copying from and to? Copying a tapped Time Vault onto an untapped artifact is game-winning. You can also net 1-2 mana by turning a Mox into something like Mana Vault, Mana Crypt, or Black Lotus. But these seem more like ancillary bonuses than strong reasons to play Saheeli in the first place.