Your question is a very odd one, think about it (or perhaps slow down and think a little less for a minute).
Are your future currently-non-existent plants going to use/need every single square-inch of those 3,600-square inches, or maybe your going to allow for some airflow and workspace in this grow tent?
I think the sf2000s have a recommended grow/flower footprint that would assure you they can pull off some good grow within that 5x5.
Figuring out what light is best for a grow space based on watts-per-square is, again imho, to a degree endless in a 3D grow environment. It is a good industry ballpark and excellent on a 2D layout/plan (as I can say without giving your question more than 0.5 second of thought) any single 600W LED fixture will crush a 5x5 and you will never need nor want more (see: overkill).
But, I think you can start with what you got.
It’s all so situational.
I think there is a wealth of help and info here for you to have, but I’m going to suggest that you ask your question again. What the vision you’re seeing for your grow space? For your needs/desires/preferences? Are you going for 3,600 solid square inches of SOG? Are you going for mainlining 2, 4, 6, 9 plants for some monster sativa-dominant colas?
Somebody here will come along and say what/anything you want to hear, but at this point nobody knows if you really need a new 600W fixture or a couple 60W side-lights. Smoke a bowl, visualize your goal, and share with us your new questions please.