I don’t have the data sheet in front of me, but I’m thinking you may have made a mistake in your math. For 8 cobs anyway. 6 doesn’t fill the driver that way.
Running 4 at 2100 ma is whatever it is. But as soon as you cut the current, your efficacy will go up. Looking at a per cob basis, you will have more lumens per watt, just less Watts. But the overall wattage will still stay the same because you have double the cobs. So double the cobs and increase the efficacy of each cob should increase overall luminous flux.