You can’t retrofit LED bulbs into halogen reflectors, not only are they illegal, the beam pattern is wrong and either the lights will be crap, or you’ll blind every one.
Philips does LED replacement bulbs designed specifically to work as a replacement for a Halogen bulb – X-tremeUltinon LED in H4 and H7 – and while they’re not road legal, my experience is that they give a decent beam pattern and the same cut-off as the original bulbs, but more light. Presumably that’s because Philips is an international company with a lot of resource to devote to proper product development As a result they work properly, but are also really expensive:
https://www.powerbulbs.com/product/x-tremeultinon-gen2-led-headlight-bulbs-h4-twin
I’m not suggesting anyone buy them, but they do, genuinely work. I wouldn’t touch cheapo amazon / eBay LED bulbs with a barge-pole. Ditto HIDs.It’s all about where the light source sits and its dimensions, if it doesn’t accurate replicate a halogen bulb filament, it’s not going to produce light from the headlight in the same pattern. Presumably that’s what Philips has done.
It also depends on the original headlight design. The simple H4 Hella headlights on a Mk2 Golf work well, the same bulb in the godawful standard H4 headlight is still pretty bad.
The problem with the uprated halogen bulbs is that they don’t last as long as standard halogens – Powerbulbs will only warranty the Philips mega ones for six months as a result – though they’re significantly better ime. Bear in mind too that halogen bulbs tend to get dimmer as they age, so even a new, standard replacement is likely to be brighter than the one it replaces.