I notice that the optimal max population is actually really close to 25... I think just chopping 1 from a bonus somewhere, like Gaia Special will put us at 25.
Don't forget all the current growth bonuses are just stand-ins. They don't scale to planet size, and probably need to be changed.
Should most of the bonuses related to different metabolisms be in separate "branches", or should there be some interdependencies?
In general, i think making clever interdependancies at this point is wasted effort, since it makes the tree harder to change, and it makes the tree harder to use for testing. Intertwining techs branches is IMHO better considered much closer to 1.0
We probably want to have some techs that increase population capacity for individual metabolism types, as well as techs that work for all species.
I'm still sorta on vacation. (thus my brief, incomplete answers) Plan to be back in full swing sometime next week.