1915.01 gallons maximum on the grow beds filled at perfect level to the top with no gravel. 1755 gallons if filled 1" below the top with no gravel. Assuming the gravel takes 60% of the volume that leaves 702 gallons of grow bed that will fill with water. You will likely need 3x the grow bed space to maintain a ratio of roughly 1:1 in your system. pH will play an enormous factor in the required ratio for you system; so, don't push the minimum limits too hard because a pH fluctuation can kill your fish in a short amount of time. The same goes for temperature.

I couldn't see the pump (website down), but I prefer pumps that pass larger solids for gravel grow beds. The efficacy of bacteria to decompose waste matter is positively correlated to the surface area to volume ratio of the matter. If you have worms in your grow beds, that's not as important. But, check the wattage on the pump and make sure it you get the extended warranty. Make sure the pump is okay for continuous usage.

How long should it take to fully circulate the 2000 gallons?
It depends what you are asking. To determine how many gallons circulates in an hour, you just look at the GPH (gallons per hour) of your pump. If you mean how long does it take for the concentration levels to decrease to 0 based on 100% biofiltration efficiency and assuming no increase in the fish waste, the mathematical answer is infinity. Although, from a practical standpoint it would only take a few hours based on a GPH equal to the size of your system's water volume to get the levels to acceptable numbers or even undetectable levels. It's an exponential function of decreasing magnitude.

How much produce do you typically get per square foot of aquaponic grow-bed versus soil?
Depends. You could get worse than soil if you have no idea what you are doing!

Numbers have ranged from 100% of the production, comparatively, for the same plants on the same surface area in soil to numbers higher than 30,000%. Yes, that's the right number.

So many factors will affect this. Aside from the usual, sun, nutrients, temperature, cultivar, species, etc... you also have how well you design the use of your square footage. It is really quite variable, but in general you can assume a greater production per unit surface area. Vining fruiting plants typically have the greatest increase in efficacy from a comparative standpoint. That's obviously because they make use of their vertical space. We live in 3 dimensions after all, and it's misleading to compare based on surface area instead of volume used!