I believe that pulling oil out of the ground is way less carbon intensive than growing the necessary amount of corn, and I believe plastic is made from some tiny fraction of the petroleum that isn't used as fuel, so the petroleum would be extracted anyway, but the corn would not be grown anyway. I'm assuming the actual production of the good itself is similar in either case.
There is also a land cost. I'd be surprised if we determined that plastic is better than bio-plastic, but until we are making bio-plastic with a low land footprint land will continue to be a factor here.
I think the issue isn't emissions in that case but a "amount of land space necessary for landfills for traditional plastic" vs. "amount of land space necessary for plant growth for bioplastic."
If the bioplastic growth land takes up more space by a certain amount than is used to dispose of regular plastic, that's presents its own set of issues.