I try to never use green screen, my rule of thumb is:
a ) If the planned background is going to be brighter than subject, shot on white, and multiply background over subject.
b) If the planned background is going to be darker than subject, shot on medium gray and put background on top in hard light blending mode.
That said, I found myself several times in situations where the art director had no clue about what the background was going to be..... in this cases the best plugin I ever seen, but that sadly is out of production, is Ultimatte advantedge. It is a nice piece of software, a little stepper to learn but with solid foundations coming from the video industry. Great spill remover, nice hair differentation etc...
There are a lot of others plugins in the industry ( Digital anarchy primatte chromakey, Vertus fluidmask, Cinematte, DFT Zmatte, Recomposit..) but I can't say for sure they will work with such big files. The only other plugin I used was Knock Out, another great software coming from the video industry that was destroyed by corel. It is still a great tool but since the last versions of photoshop I find I can go on without them.
Yet, the best way to get rid of chroma is still photoshop manual work with channels and blendings. But we all knew that didn't we?.
