I think that you have more things covered than folks on this board with several grows under their belt. At the same time it is always a learning exp. As far as the fans I use a 172CFM vortex with a 90CFM fan on a 1K and Temps are fine in a area that is smaller than 4x4, but I have made other modifications and have the vortex sucking the heat right out of a glass enclosed hood with the 90CFM pushing air into the hood at the same time. The vortex sucks the heat not only out of the hood, but out of the cab and pulls it threw my scrubber. Have you thought about making some cool tubes? It will improve the whole grow not only because it can eliminate heat probs, but you can get the lights allot closer to the plants. IMO those 400w's need to be no further than 18in an even at that, you are starting to loos lumens, ideal is 12in and under.
And those breakers will be just fine, technically you could runn 3 400watters on 1 15 amp. I have my 1K and a few fans on the same breaker as the washer and dryer, given I don't make it a practice to run the light at the same time as the washer and dryer, but the dryer has been turned on a few times while everything is going. 3 400watters would push it a little, but remember a 400w takes about 3.9 amps at 120 volts, you shouldn't have any problems in the long run. Use power strips that have GFI circuitry so they will trip when over loaded with more than 15 amp.
Hope all this helps, and like one folk said, don't tell anyone, it may seem like a fun pastime, but sitting in a cell aint. Also I really wouldn't put any other plants in there do to any insect problems, some do it so feel free to, this is just my, YOU KNOW.
Adieu