After much consideration...
A game I'm proud of:
http://councilroom.com/game?game_id=game-20110714-105020-05239e58.html
Bad end of a 7-3 curse split! Saboteur play! I was at -2 points at the start of my 2nd to last turn! Doesn't get much better than this. You can call me "Mr. +2.91 Horn of Plenty Effect with."
This game in mind I could yesterday put into practice the following comeback (again no + buys):
http://councilroom.com/game?game_id=game-20110731-144148-1ebeeb10.htmlStarting 5-2 on an ambassador board with Horn of Plenty as only 5 and without 2$ cards I gambled a bit at turn 3 returning 2 coppers instead of 1 and not buying silver (probably a mistake). As a result I had 3 Estates and 2 Coppers at turn 2, 4 and 5 (!) whereas opponent bought first gold at turn 6 having already 2 caravans.
I still wasn't ready to buy gold after turn 12 when he had 3 Golds, 1 Platinum and 1 King's Court. Probably I made some more mistakes and should have bought a second silver earlier (at turn 9 or at least 13, but the start was really frustrating and I wanted the gold right away. After turn 12 however I didn't even have enough copper to do so):
http://councilroom.com/game?game_id=game-20110731-144148-1ebeeb10.html#dza-show-turn-12Agrisios: 2 points : 1 Horn of Plenty, 2 Caravan, 2 Ambassador, 1 Silver, 2 Estate, 3 Copper
dza: 3 points : 1 Platinum, 1 King's Court, 3 Gold, 4 Caravan, 1 Ambassador, 2 Silver, 3 Estate, 9 Copper
After turn 17 he had already 4 Colonies and much more buying power. But I had this Horn of Plenty which I used this turn to gain a curse and than it began:
http://councilroom.com/game?game_id=game-20110731-144148-1ebeeb10.html#dza-show-turn-17Agrisios: 0 points : 2 King's Court, 1 Gold, 1 Horn of Plenty, 5 Caravan, 2 Ambassador, 2 Silver, 1 Wishing Well, 1 Estate, 2 Copper, 1 Curse
dza: 42 points : 4 Colony, 1 Platinum, 1 King's Court, 3 Gold, 5 Caravan, 1 Ambassador, 2 Silver, 2 Estate, 11 Copper
Funnily bureaucrat brought the horns to 11 which probably wasn't really necessary. There are some more things perhaps like using the only King's Court for a duration before reshuffle.
Conclusion: There seems to be a pattern here. You really need at least two Horns in these setups with strong enablers like King's Court to be safe no matter how big your advantage is.