I think tanks will pretty much out DPS most DPS specs until sometime into level 70-80, at least that's been my experience single and multi-boxing. On my lowbie solo-tank that I'm playing with a friend I am hands down at least 50% of the overall damage in a team and usually first in actual DPS by a very wide margin.

The only problem I would see with 5x Druids would be a lack of buff diversity. Druids don't really stack well compared to shaman in that regard. But, assuming you can put together a decent rotation for the DPS, a 5 Druid team would be extremely viable. And you could stealth which is nice.