First, the supernatural sense would be a bit much for my taste (pun intended
). For him to know where things are from just by tasting it should not be part of the power. If however he knows about the stuff he tasted, he can then identify the origin by that knowledge, but not by tasting alone.
For example: "This wine seems to have slightly more iron content than other wines."
If he knows about it, he could then deduce, that: "There is one vineyard that has unusually high iron content in the soil, so the wine must be from there."
There are people that can do that without a supernatural power with wine, but I hope you know what I mean by that example. Knowing the origin of something by tasting it would be, in my opinion, a power of its own.
As for the physical immunity:
I would go ahead and count like this:
It only protects against a specific thing [+2], anything else is easy to come by [+2] and finally anyone could easily find it out (for example by doing anything that is not trying to kill you via poison) [+2]
Which brings you to a cost of -2 for the power, which I think is a fair price for this and the kinds of shenanigans you can do with this. (Inconceivable!)
Or you could argue that:
it not only protects against one specific type of damage, but also against a specific type of exposure to that damage. Injected with a poison your character would be just like anyone else. So that would be [+2] for only poison and another [+2} for only ingested. BUT in that case I would put the knowledge of the catch to "must personally know you", so you don't get anything from there and would still be at -2 refresh cost. Like I said above, I think that is not too pricy.
Of course there is still the question of what is a "poison"? Powers like this tend to block out anything that is ingested and not just food, especially medicine or in the case of your alchemist, he might be immune to his own potions. So you might not want to aim too high with the rebate, because you might end up with a bargain on your superpower, just to figure out, that it was not, what you bargained for.