That's interesting regarding the tank size, on their website it does say they have a spread of 24" x 24" with a peak PAR of 100µMol at a depth of 24 inches, but this figure is taken in air and not through water.

my tank is approx 20" x 16" but has a depth of 20" but as the measurement for the HD prime was taken in air i'm just a bit concerned that if i wanted to house some more light demanding corals they might struggle.

I do like the fact they have 7 channels so you have more control, as for the mount, i have seen some have had issues with the light drooping with the solid tank mount but haven't heard anything negative about the goosenecks.