Transmute here is competing against a couple of other very strong options:
Silver/Mint burns 4 (or 5) coppers so you can just mass Mint silvers/golds sooner. The silver VP will wrack up quickly ... but you can take an early Fairground to nab some of them with not too much tempo loss. Delaying the Mint until you can take out the potion has the trouble of needing to hit $4 in 4 cards. You either lose a silver (or gold) or need to wait until you have Market or Baker online - that is a lot of time drawing coppers. Say you play the Transmute once ASAP and bought silvers on the other two turns. Your total cash in deck is $14 over 14 cards. By T5 a Mint/Silver opening should be looking at three silvers most of the time (buy one to start and either buy one/Mint one, buy two, or mint two by T3). That gives you $9 over 10. That is pretty close and the Mint will be getting more reliable while the Mute will be getting less. Using the Mint to thin down after that requires either burning a silver (undoing a good bit of your thinning) or waiting.
The other opening is Th/Silver. This can allow you to trash more coppers (5 being easy, 7 unlikely) and have a stronger card at game end with quicker +buy (e.g. getting Baker/Pawn). Again it is a LOT of turns before Transmute gets your deck that thin.
By the time you start buying estates for VP chips, I suspect the other guy will just have nabbed some green of his own.
Burning a full two turns on getting Potion then Mute is just so slow. I would be much more tempted to try the Mute run if I could cut that down a bit. E.g. something like Gear being out so I can hit Gear (return 2) -> Th -> Draw up to 9 cards, gain a Pot, buy Mint. Mute is massively better if the cost of acquiring it is not burning two full turns.
Particularly with Fairgrounds out, I am not sold on needing to burn the Pot afterwards, I need 15 Uniques in this game. Assuming 6 from the stock cards that means I need 9 from somewhere else. Eventually I come down to Curse, Th, Ms, Cb, and Pot. Th is terrible if I plan on buying green to Mute or going copper -> Mute -> Duchy; better to spend a $5 on something else. Cb and Ms are both not bad, but do require me to forgo a duchy or maybe a gold buy; I suspect it will come down to shuffle timing of do I get both (trash the Pot), neither (keep the pot, gain a curse off a spare buy), or just one (keep the Pot). Again tempo matters a lot, granted less with equivalent Prov/Fg piles, so I am not sure that waiting to kill the Pot is worth it.
I have actually considered calling Th/Mute a viable combo for engines. With an engine you can gain the pot, draw it, and get the Mute all in a single turn without sacrificing your main buy. You trade a gold now for three later and can even Mute the pot to a Duchy or Gold. Mute is vastly better in an engine as you have vastly better odds of lining up Mute/Green and can be sure for a while that buying an Estate with a spare $2 will turn into a gold next turn. Mute is a vastly better card (still in the bottom quarter) when you can gain it for less that giving up two turns worth of buying power.