Photosynthesis occurs across a much wider range than is visible to the naked eye (the wavelengths that PAR is constrained to).
For some clarity on the UV front, UVA is typically considered to cover 320nM to 400nM, 290nM-320nM is B, <290nM C). MH bulbs can put out a bunch of UV, not limited to A. That is one reason they will often have very thick tempered glass between the bulb and the tank (plus, they have a nasty habit of exploding if they get wet while on - the glass responds to the temperature differential and shatters).
UV light is very high energy, and can bleach coral (and burn your skin) if too much is provided, or introduced too quickly. On the plus side, a lot of the caratenoids and xanthophylls that are used for photo-inhibition (protecting the coral and its symbiots from excessive light) are also the ones that give corals some of their best colors. Some caratenoids are also photosynthetic.
The reason MH guys are adamant about MH is that the broad spectral output will bring out a ton of colors in corals. The logistical challenge is that you get widely variable light density depending on where the coral is relative to the luminaire - MH act a lot like point sources, and so the output drops significantly the farther away you get. They also drop in output very quickly because of how hot they run, so if you keep a high light requirement tank (say, all Acros and Montis), you will need to change them frequently to keep the light environment stable. Plus, the heat - it has to be diffused somehow, usually by loud-*** fans, and no matter what you do some will wind up in the tank.
LEDs are a lot more advanced than they were 20 years ago. Blue and white were the easiest and most efficient to produce in the early days, followed closely by the mid-600nM reds, though that came later. The whites have a big blue spike, which is why they look metallic. However, with the $B's spent by companies like Phillips and Nichia, they have come a long way, so you can generate a much broader spectrum with LEDs than has been the case.
IR is beneficial to the propagation of the zooxanthallae themselves, which indirectly benefits the corals. Part of the reproductive process of all plants, marine or terrestrial, is linked to IR in the 780 range and the 840 range, though the latter won't be seen in many reef lights - they are expensive as hell.
For my money, I'd rather drop some extra coin up front, and reduce the amount of maintenance and the "where do I put this new frag" dance, and get a solid light that will last for years and gives me a uniform light palette across the tank. Check these guys out.
https://acrooptics.com/
Pricey in the sort term, cheaper in the long run, and they have a very long run. There are quite a few LED lights available that skimp on the heat diffusion in the name of slick design - even with the much more efficient current-to-light output compared to MH, LEDs do get hot, especially right at the pad - I've seen high-priced Radions burn out in a year, though run correctly, the diodes they use should last 10. I have one of the Gen1 AO lights over my SPS tank, and it has been humming along for over 5 years, putting out the same amount of light as the day I put it up. (I am lucky to have a Licor 193 submersible photometer to keep track, one of the benefits of being surrounded by a ton of engineers.)
Back to the initial question, 300 PAR in a tight wavelength pattern (especially 440-470 blue) can be overwhelming for corals, simply because they penetrate water very efficiently - that's why pools look blue - it will bounce of the bottom and make it back to your eye, where the greens and reds will not. So, I see a lot of tanks that keep the PAR reading low, maybe 150-250, where the corals are happy, although they look like windex. Take one of the corals out and place it under a broaer spectrum light, though, and the bright colors you see are no longer visible. Stick with a broad spectrum light, whether MH or LED (or T5, or whatever). Your corals will thank you for it, and you will be glad you did. There is a lot more to attractive, thriving corals than just photosynthesis.
or harder.. depending..
