Nearly nobody needs 400w MH unless you have serious light-demaning corals or are looking to spread a large area or penetrate super deep. I am a super serious Acropora keeper (no Montis, Stylophora, Birdsnets or Pocilipora) and I use 250w halides. 150W HQI Halides have lots of output and are lower in wattage and a great choice for mixed/LPS tanks with superb color and coverage - three of these on most of the time with no T5s is only 450w and you could grow nearly anything anywhere. You do not have to use the T5s all of the time. I do not use any at all - I don't want to pay for the wattage and I pick MH bulbs with the color that I want so that I don't have to supplement them. Some use the T5 only for dusk/dawn and shut them off when the halides are running.
Watts are watts. There are no efficiencies with LED - 250w is half of the power in the tank of a 500w fixture... do not believe what a manufacturer tells you. There is more to it than this with a bit of wasted energy over 850nm for fluorescent and MH bulbs, but the peaks in LEDs are just as inefficient. The spectrum from 350nm to 850nm has value - all of it. UV has huge effect on proteins and pigments of most corals and IR over 700 helps the corals process energy by relieving pressure of PSI through PSII - the presence of IR is why you can use more of a light containing it and corals will grow faster and not get harmed. Nearly no LEDs cover this entire spectrum, but some are getting closer - some are using 390nm true UV diodes now (now quite down to 350 yet) and the European panels are putting IR into the units in good numbers. MH cover all of this spectrum in spades - T5 do as well, but each bulb will have more or less depending on temperature (actinic more UV and less IR whereas daylight will have less UV and more IR). There is a great thread on the other board about 6.5K bulbs that evolved into a spectrum thread that might be worth a read - the results from it have a lot of LED users considering 6.5K T5s to supplement specifically for yellow, green, red and far red that diodes don't appear to get to yet.
No, you cannot have too much good light to harm corals. However, they might not need that much. I keep a few LPS and softies in my tanks and while they might not need that much light, they are not harmed. You can indeed have too much bad light. Lots of white diodes in LEDs can be harmful with peaks and spectrum that burns coral, which is why you see so many at 50% all of the time - this does not happen with T5 and MH.
Photon V2 is a fine LED for most mixed tanks all the way up to beginner/moderate SPS. It covers better than a Radion or Kessil, but does not quite blend colors as well. Even though I have never used one, it appears to be a nice blend of price, performance and features. Nobody runs these at 100%, so your wattage will be lower than the max. The corals do not appear to be able to handle the white spectrum when turned up too much - the corals can get harmed. The Photon V2 is in a nice place between the super-cheap black-box units and the more expensive, ultra featured, wider spectrum Atlantic Orphek v4, for example.
