Is it possible to use more than the allocated base amount of video memory?
It supports up to 512MB on 13" models.

I'm afraid if your model is 15" or 17" there's nothing you can do.

Oh wait, your last chance is to upgrade (coming this fall) to Mac OS X 10.11 El Capitan which includes Metal, a new graphic framework. I have El Captain Beta installed and the allocated GPU memory went from 1024 MB (in Yosemite) to 1536 MB.