In 5/2, would you take a mountebank even with familiar on the board ? I know Mountebank/Chapel is the best opening in Council room...
I think twice about going
Familiar with every other Curser on the board. With 5/2 and Mountebank, you have to wait an entire more shuffle till you can hand out the Curses with Familiar. Same with Sea Hag (might even discard Potion, or lead to $2p for the Familiar). And you have to buy a "Curse" for $4 the first round. And you should better not get $2p in the first shuffle. Or see the first Potion in turn 5.
So there is so much against the Familiar, and starting 5/2 I think I would not even think about the Familiar.
Simulator (just 1000 games each, but I think the pattern is clear):
MB-Familiar: 87-11 (91-8 from 5/2)
SeaHag-Fam : 91-8
Witch : 91-8
YW: 93-6 (but of course the Bane might change this)
Even unsupported Torturer wins against Familiar. Though the built-in Familiar-bot is not optimal, it also buys Fam>Silver when the Curses have run out, but changing this doesn't change much.
Edit: Even if you click through some of the other BigMoney-bots, many of them beat the Familiar.