Not true. I added some LEDs and I'm sure that helped some, but as I said I have quite a bit under stock lights. Think about it, 2-5 watts per gallon is what most corals need, and that's 3 watts per gallon default right there. Mushrooms like less light, as do some others. I have Palys, Protopalys, Zoas, Sarcothelia, Pachyclavularia, and Montipora (at the very top of the tank, granted) with relatively little upgrades.
No mushrooms, but things like Actinodiscus prefer less light, which is why people with metal halides usually put them in shaded areas of their tank. They are 10000k lights, so they do support corals fine. You could go with the 28g HQI one for about 500 dollars, but you may need a chiller if it gets hot in your area (like it does mine).
Not all corals require intense lighting as you may have been told, just add more blues through LED
strips to get a better appearance
There are beautiful corals that require no light at all, though these are challenging to keep. I hope one day to have a tank of Sclero and Dendroneptheas, IMO
the most beautiful of corals though they are the most challenging.