I do think the comeback mechanic is a bit too extreme, in extreme cases. When you're leading by 15-20k and one of your cores die, the enemy team gains too much money for a single kill I think.
I'd probably alter it in 2 ways. First make it scale more or less based on the time. Early in the game, less, later, more. Point of this is to reduce the whole "ok guys it's 20mins, we dominated early game and pushed every tower, now let's farm for 15mins until we can finish". If you get an early advantage, you should be able to push and finish the game without running into the risk of giving it all up in 3deaths. You shouldn't be able to throw boddies at the rax until they fall like you used to, but still.
The second isn't really a solution but it'd be a way to not punish initiators when you're leading. Currently if you're 20k ahead and try to take a fight with say Clockwerk or Slardar as your initiator, and he dies after initiating, leading to a 4man wipe for only his death, this should be a victory, but you're probably breaking even on gold. This actually makes it less appealing to pick certain heroes that often die to force fights because they're disadvantageous in those situations where you get ahead early. I was thinking of comeback bounty bonus being based on kill frequency or something like that, but it'd be abusable by playing passive and shit so not such a good idea.