Play with any cheap 2 channel led fixture. If you run the white up to full intensity (simulating your MH lamps) and then start adding in blue slowly, you'll notice there is VERY little difference in the look of the tank until you've added a LOT of blue. Even at 100% blue the overall light has only gone from white to a bit cooler white.
It also matters a lot as to the kelvin rating of the MH you are pushing to 300w. A warmer (6500K) MH will be affected more with the RB leds than a cooler (20,000K) MH will.
But either way, I don't think you'll see much 'pop' even with 1.21 Gigawatts of RB leds as the fluorescents of the coral due to the RB leds will be so washed out by the white of the MH.