There's a lot of benefits from taking longer to develop a game using the feedback of a large base of testers. From past experience in Stardock betas I've always felt that the community had a lot of positive, creative things to suggest. There will of course always be suggestions that are unrealistic, but that's part of the fun.
No financial pressure to release is a pretty rare thing and it's often said that they never finish their games, they simply stop working on them. As long as the time is spent in an efficient manner and there's a plan that gets followed, I think you should definately extend the development process. Just keep on target and don't get seduced into implementing too many things which time might not allow.
4x games always have such grand potential, since they can be expanded in loads of directions (tactical combat, dungeons, empire control, aesthetic detail etc) and giving the game time to mature in a slow manner will definately highlight what's missing or what doesn't feel up to quality of the other aspects.
I also like the fact that playing from the cloth map in the initial phase of beta will allow you guys to try new features without having to make a lot of assets that might be trashed shortly thereafter. More time = more testing different things to fine-tune the gameplay.
I'd say go for it, Stardock has always been great with betas and I think the community respects and trusts that enough for you to do whatever you want
Underwater cities and spells that shatter the world, forcing players to use steampunk airships please. Ok I'm done...
EDIT: Look at how Blizzard is working on Starcraft 2, they are taking as long as they need (they certainly have enough money to work on it) and have been adding new things/rewriting old things and tweaking the game based on feedback that they are getting. They've been working on the game for a while and they certainly have a lot of pressure to make it good, but it's a perfect example of how much a game can benefit from a long testing/feedback phase.