I don't think it is very good at all, at least generally - I am buying potion (junk card), feodum (junk card), and transmute (junk card), so that IF the transmute collides with feodum (not likely in the general case), I can get 3 silver and a gold, still have two junk cards in deck. I imagine you are better just buying treasure.
Now, *can* it be okay? Sure, if you already have the potion, and you are building an engine which can accommodate the higher number of stop cards, and you have a spare buy where you don't need to get a different potion-cost card (this is a lot of ifs, but it can happen), then maybe it works out fine. I expect this to be rare.
Certainly I expect vanilla BM to outperform it in the general case.