If you don't have any deck spying, then the mystic usually won't be drawing a card, so it's usually just +1 action, +$2 for a $5 cost. As long as somebody has trashed a copper, the forager is at least +1 action, +$1, +1 buy for only $3. If anybody has trashed a better treasure, then the forager will be +1 action, +$2, +1 buy.
From a theorycraft point of view, shouldn't forager be way better than mystic on average?
Not at all. First, you say Forager is at least +1 Action, +1 Buy, +$1 "for only" $3, but this is a terrible $3 effect. It's essentially Copper-as-Action that gives you +1 Buy. It would be a poor card even at $2, though you might pick it up at that price occasionally. And you'd buy it at $3 or even $4 if there were no other source of +Buy, but you'd have to be truly hard up for +Buy if you'd prefer this over Silver.
And a Silver is basically what Mystic's minimum effect is: a non-terminal but non-drawing source of +$2.
Now let's consider what these cards look like on the upside. At best, Mystic is an activated Conspirator. This is about a $7 effect for $5. (Why $7? For a few different reasons, the best one being that this was the initial draft of Grand Market: no +Buy, no Copper restriction, and a $7 price tag. Similarly, you can argue that Peddler is balanced at $4, so the same card with an extra coin on top of it should cost $3 more, given the price differences between the base Treasure cards.)
Forager can become better than this, but it takes a lot of work, trashing valuable cards like Gold. It's better with weak kingdom Treasures (Loan) or self-trashing ones (Horn of Plenty) or ones that trash other ones (Counterfeit), but outside of these combos you probably don't build it up beyond "Silver-as-Action-with-a-Buy." Occasionally something will trash a Gold, so then it's Gold-as-Action-with-a-Buy, which is pretty good -- about a $7 effect! Which is what Mystic can be more often and right off the bat instead of only by the later stages of the game.
Admittedly, a lot depends on how often you can activate Mystic. In heavy DA/Cornucopia kingdoms, this will be less often than otherwise. But I believe (without having played with it yet) that it will be easier to activate this than Wishing Well, just because it will work with a lot of top-decking combos that Wishing Well does NOT work with, such as Spy. It also self-combos (one failed Mystic causes the next to succeed) whereas Wishing Well does not.
So I definitely see Mystic as the stronger card on average. That said, Forager has the better upside in certain kingdoms, similarly to how Trade Route can dominate in kingdoms with lots of Victory cards. Even then, though, it'll definitely be tougher to build Forager up than Trade Route, because buying Victory cards to activate Trade Route is stuff you eventually want to do anyway, while trashing Treasures to activate Forager is more often a penalty.
