I moved from 8 remote mounted driver LED COBS to 2 remote mounted ballast 315 CMH in my cabinet (plant space roughly 3' x 5").
In that space, the CMH runs about 3-5 degrees warmer than the COBs did, but the quality increase due to the spectrum difference sealed the deal for me.
That said, each of my COBs was the same temp and I think a mix would have performed better. I could have used another 2-4 COBs in there as well.
A third CMH would be insane in there, but they are so cheap I may do it anyway.
Quantum boards may be different animal than COBs though - do they combine different temp chips on the board or are they typically all the same?
Gotta be a game changer if Joe likes em