Each of those lights is supposed to actually draw 270Watts, not 1500Watts... so you have a total of around 550 Watts.
Now, this is not the best reference point, but with high quality, latest generation white diodes (samsung, osram, etc) you can usually have very good results with around 30 Watts per sq feet... you are at about 27 Watts / sqf with those lights but they aren't as efficient as latest generation white diodes, so you might need more light for optimum results with those purple leds.
BUT, I would go ahead with that configuration before adding more lights (unless you have money to spare.. but in that case, get better lights... no more blurples) and give you current configuration at least one run to see the results... it might not be as bad as you would think... "optimum" results are not necesary for everyone... i for one am very pelased when i get sub-optimum results... i get very good quality but just not as much quantity or big buds as others would like... but still my buds end up costing me way less than buying them so win-win for me.
that would depend on genetics and how you plan to grow them (hydro or soil) and pot size and if you will train them or not. don't have much experience with autos though, so not sure what to really say here. In my 2x2 i can fit two small autos or 1 big one... but i prefer just to have one while i dial stuff in better.