Actually, from the sims, it seems to me that coppersmith+BM is not more effective than BM. Well, maybe 1% more effective on a 5/2 split, but actually a little worse otherwise.
WW's intuition is correct (barely)
I just modified WW's Ultimate BM bot - added a single copper smith, and it loses 46.6 - 45.2 - 8.2% on Ultimate Sim My intuition was that the coppersmith would enable early gold buys slightly more often, which seems to be accurate, as money generated is higher early, but lower late.
It looked like it ended up buying provinces about 1/2 turn too late. If I dropped the coin in deck condition to 17, it got slightly closer, but still lost. If i dropped the coins required to 16, then it was buying provinces too early.
It looks like it does better early, but if you end up drawing the coppersmith with only 1 copper late in the game, it's is a killer.
Buying a copper on a 5/2 start was a disaster.