Thrones Feast to get two, then keep throning Mint for more coppers/silvers? If you can line then up, that gets you more cards than +buy.
Lining up TR-Feast has the same odds as lining up two Treasure Maps, so not a great thing to be basing your Gardens strategy on. And in that case, would you still open Feast? Maybe, I guess...
The other thing is, if you successfully TR-Mint a couple times, you won't be able to keep doing it. The extra Coppers and Silvers will make it harder to collide them; your TRs will be dead often.
Is there a time when you prefer TR-Mint with silver in hand to TR-+buy with silver in hand? Imagine its these terribad kingdoms. I.e. you throne the +buy but all it does is get you 2 coppers in addition to your regular buy--the Mint got you 2 silvers plus regular buy.
It would depend on the +Buy card, but if it would get you no extra money (i.e. you would only buy Coppers) then yeah, Mint would be better (barring edge cases like Counting House). But the benefits of Mint would not outweigh the opportunity cost of getting that Mint in the first place, I think. I would much rather open the +Buy card and get to use it after the first reshuffle than have to Feast for a Mint that I won't get to use until after the second reshuffle at the earliest.