Wow I typed a long response yesterday to this thread but FF crashed out and it didn't post lol. The just of it was:
LEDS are not going to come near the output of your hids. The reason your "fog lights" are ineffective in fog is because you don't have fog lights... anything higher than 4k is essentially pointless in the fog and can do more harm than good. Its like driving with your brights on which we all know your not supposed to do in fog.
Many members will call your lights ditch lights because all they actually are good for is lighting ditches. Even if you went to leds that were as bright as your hids and spent all of that money because they wont be cheap, it would be pointless and your fog driving would not improve at all.
If you want lights that improve your line of sight in fog then get a set of 3k hids. Yellow is the most effective color to cut fog. If you just want your fog lights for looks then 5k+ is fine... but if you want them to do what they are meant to do then go 3k.