Yes, I understand that, but that is why you save the money. You do not have to run as much COB lighting to match HPS, which is where the savings is at. Either way you look at it, you are saving. Although, at 1000w in a 4x4, you are pretty much spinning your wheels and wasting energy with COB. The yield of 1000w in 4x4 compared to 600w in 4x4 is small. I have never yielded less than 1.2gpw in a 4x4 with my COBS @ 600w. Very few people can even achieve 1gpw with HPS (flame suit on for all the turds out there who claim 1gpw every grow with HPS when in reality, its uncommon... Stop measuring your plants on the vine!). I know that I can yield a bit better if I added 200w, but I am pretty content with yielding what I do.
You are now getting into space limitations. If you dont have limitations, of course 1000w of cob will blow away 1000w hps and what you are saying is 100% true. I am just using a common 4x4 setup as example.