your story reads a lot like mine, my journey though the HPS to LED's. only when I started changing out lights: w/the UFO and dozens of grow light companies.
it wasn't until I started to mix lights when I got my kind K3's a friend loaned me some CMH lights to help flower, he hated the KIND lights, and I was constantly messing with them: timers light intensity, spectrum changes. anyways it was then yield I saw, I could play in the same school as HPS.
so I dumped the K3's and got some spectrum kings, and along with the CMH's lights was now in the same grade as HPS, just outside1/2 elbow of my old yields.
I have been pretty happy there, as the work load, and ease of running my solution made the loss in weight acceptable, and it was no longer been noted as "different" so it was growing good flowers.
now, I'm playing with some COB's and I'm still mixing my lights, I know I'm just about 6 cobs from 2250/1125(2) HPS yields. with a total of 15 cobs and 2 CMH lights. thats only a savings of about 530 watts, but that included with no more lamps, and hard working A/C's its better.
I'm seeing over the fence in a manner of speaking, I'm guessing about an all cob solution would look like 15 cobs per 1125 if HPS if you do the math, its pretty close to the same pull from the wall. youll save on cooling at least by 30-40%(guesstimation)
far as yields it varies widely 1.5 to 4 per 1000 watts, depends on all other factors. but, can you replace HPS w/COB's....YES, yes you can. everyday there are great grows, that would only a few years ago only be expected from high watt HPS's at a faction of power.
I think you might want to look in to mixing your lights with other low power sources of PAR.
things are moving pretty quick though, keep reading boring stats, and see what the people are doing here on RIU, just saw a thread with a guy running like 140(20-30??)watts on a 3590s with very impressive floor data.