narrowing down the possibilities by a factor of two is always one bit of information

Provided, of course, that your space of possibilities is finite and you know what it is. In the case of evolution you don't.

that's equivalent to saying that if you order all possible lifeforms

I don't understand what does "all possible lifeforms" mean. Does not compute.

but there's always a limit to the information, and I'm pretty sure that it's pretty much always a limit that's already been hit.

Which limit? The limit of information in the mammalian genome? Or the limit of evolution -- whatever exists is the pinnacle an no better (given the same environment) can be achieved?