If you feel like you're 'getting sniped ' by other healers being highly proactive and on-the-ball, then that may mean your group needs one less heal in the group and one more dps to make the run go faster and more efficient.
With healer output is the one 'output' in the game that's determined equally by what other healers are or aren't doing as it is determined by what you are doing. Healer output currently is absolutely freeeeking insane and if you have healers that can just about solo heal ten man or 4 heal 25 man, even on some HMs. Go for it! The more DPS you can pump the less time they have to be worried about what's going to happen to the group and the faster you can get loot..

in addition, if your healers are so bored because all they're doing is overhealing, they're going to lose their edge when it comes to difficult encounters later on. If you have so many healers in the group that you aren't giving your healers a challenge, then no only are they bored but all they're doing is spamming a single heal because they don't have to think to do anything else... don't want your heals getting lazy. ^.^

Personally, if I'm in a ten man ICC with that redunkulous 30% buff, I will refuse to join a group to heal if they insist on bringing more than two healers. I can very easily do 70% of the heals by myself.
If it's a ten man ToC or lower I solo heal everything unless a boss encouter requires more because of the strat. lol in fact, what's funny is that solo healing ToC ten is actually far easier than solo healing trash in ICC on the frost and blood wings [the only issue with blood trash is if you get sheeped by the casters 0.0].

Anyway, that's my 2c. But I guess there's no 'right' answer necissarily because it looks like most input on solutions to this issue revolve around personal play style and enjoyment.. which is good! I'm glad to know so many healers out there are passionate about their role in raids