IGG gains you extra coppers.
If you play the IGGs correctly you gain minimum copper when your opponent is not cursing you. You don't need to take that much copper either to buy 5 cost cards when you've some silver in your deck and no curses. This could of course be how my simulator IGG player gets better results than seen in the original game.
Oh, absolutely. But you'll still probably gain
some Copper. The question was where extra Copper would come from, and that is where.
On the actual question of buying Stashes first:
- To reliably get Province every reshuffle, you will need 4 Stashes.
- Those Stashes could have been IGG or Duchy.
- Why switch to IGG late if you are going Stash early? Why not just go all the way and grab 8 Stashes, hoping for 2 Provinces per reshuffle?
- Would straight Stash actually beat straight IGG?
Keep in mind, IGG is definitely capable of grabbing Provinces and also way better at grabbing Duchies, plus there are the Curses to offset as well. I don't see how Stash could even stand a chance.