Of God, not links to candlepower forum... Ok, its a great resource, but the mod of the automotive sectuon is a literary genius and uses his ability with language to force compliance with his own personal views. Unfortunately this has resulted in CPF becoming a bastion of dogma and not a discussion of merrits and drawbacks.
That said, LED retrofits are not "there" yet and as a result have poor lighting. Unfortunately the ability to "throw" light requires a very small emitter, like how an electroluminescent pannel can make more "light" but barely project it yet a small flashlight can put good light a fair distance away. Current LED tech can't make both high output and small emitters, and the larger emitters are of the wrong shape to use the projector.
As for HID kits, some work well, some are [censored], and it really depends on the vehicle. There are many kits installed in projectors that put out a better light pattern than factory HID projectors.
As far as legality goes, well nobody [censored] when someone puts h9's in place of h11's, but that isn't any better from a "legal" standpoint. As far as DOT certification, the DOT doesn't certify [censored]. The only certification is by the manufacture and they state that their headlight as equipped from the factory meets DOT specified light control. You could absolutely make your own headlight and call it DOT certified, help yourself. BUT, if it doesn't meet DOT designated light control then brace yourself. The same goes for swapping bulbs, you could put in a cheap 50w HID kit and have a DOT certified beam pattern, it just has to put light where it should, and not where it shouldn't. All you know is that that particular combination of bulb and optics wasn't tested by the manufacture to achieve a certain standard.