Your ethanol yield estimates from corn are addressed rightfully above. In addition, algae is being proposed as a source of biodiesel, not ethanol. I will believe the yield estimates once pilot experiments are scaled up.
One thing which was NOT addressed is that algae farming does not require farmland at all and is best done in desert areas. Other than that the yield differential is many to one and the idea of turning corn into fuel is still basically insane and criminal.