The addition of light is affected by the law of diminishing returns. If you put in a 1,000w light you would certainly see benefits over a single 600w. 2x 600w is overkill, and 1,800w is unnecessary and probably dangerous.

I would just use 600w and be done with it. It's only 5 plants! Better use would be to get a second tent for mothers and vegging so that you are running a tent dedicated to flowering.