I'm sure they researched this but square panels seem suboptimal. I wonder if manufacturing made that determination. It seems like circular or hexagonal panels would better utilize the space. That or just grid aligned square panels. Maybe the space efficiency doesn't matter that much? Each configuration captures the entirety of indirect sunlight. The little bit extra you'd get from direct sunlight is likely insignificant compared to overall performance.
I don't think the panels are the limiting factor. Remember this isn't a photoelectric plant; the sunlight isn't being converted directly into electricity. All the panels are doing is reflecting sunlight to the central tower, where it heats up molten salt. The limiting factor for power generation is probably the heat capacity of the salt, not the amount of sunlight the panels can reflect.
Also, the panels don't look like flat squares to me; they look curved, so that they concentrate the sunlight they reflect.
