I think I recall watching that BRS video, but was it for EB8 or EB832? The EB8 triac outlets should be fine as heater controller. The EB832 mechanical relays not fine.
The question of heat flow from the heater filament to the inside of the case, through the case to the outer surface, then to the water touching the heater, then mixing in your sump...is a bit complicated...no kidding, right. However, at 2400 gph, that should be fine IF the flow velocity at the heater is good. If the heaters are somehow in a stagnant area of the sump, then that's no good. You need good flow velocity at the heater to get the heat away from the heater and good flow volume rate to the display tank to get the heat from the sump to the DT. A possible problem is, I think, the problem I'm having, that the heat inside the heater takes a while to get through the housing so ends up heating up the heater thermostat and cutting it off too soon. My heater cycles on and off more than I think it should and even when the temp in the tank is low, the heater cuts off frequently. I have my heaters between the baffles in the sump, so flow velocity is probaly OK. BTW, I'm using two 300 W heaters in my 140 gal system. Two 300 W for you, ~ 400 gal, seems low. Another thing to consider is, what happens if your house becomes unreasonably cold for some reason, e.g. heat failure or open window in the winter. Too cold is much much better than too warm, but maybe you want some heater capacity margin safety factor.
Don't believe the temperature setting on the heater. These can easily be off +/- 2 deg. F. In rare cases +/- 5 (credit: BRSTV). Don't believe your thermometer either unless you've calibarated it, it's NIST traceable (and relatively new) or you've compared several, like 4 or 5, thermometers. Thermometers are also often off by +/- 2. I have several glass thermometers which I calibrated with ice water and boiling water and compare them with each other. I'm comfortable they are within +/- 1 after calibration. The worst is off by 3.
I don't know what daily temp swing is OK. Probably depends a lot on the particular coral, if it has acclimated itself to it, what the actual high and low is. Someone else will need to weigh in on this one. Check out
https://www.reef2reef.com/threads/how-much-can-water-temperature-swings-effect-sps-corals.358578/
If you are truly using EB8, NOT EB832, you can use the triac outlets without worry.
Put your heaters in an area that has high flow velocity. Maybe add a small pump to blow water by the heaters.
Consider bigger heaters.