I'm hoping no one is expecting foglights to improve visibility, as they do not. Foglights are engineering to have a low and wide beam pattern that shoots under the fog to light up markings on the road directly ahead of you, to help keep you on the road. The lower the visibility you have, the slow you should be going, regardless of the lighting you have.
The idea isn't to have brighter lights, rather it's to have lights effective at going under the fog to hit those lane markers. Brighter lights are only desirable if the OEM lights are too dim to accomplish that. Once you can see road markers in dense fog, they're bright enough.
I've had four different lights now and may investigate a fifth (haven't decided due to expense). First is OEM standard halogen, which we all know is crap. Aftermarket halogen bulbs will have the same effect, maybe with a different color to look cool to oncoming traffic. Second is drop-in HID. To fit, I had to remove the reflector that was covering the bulb. This produced a lot more light, however it scattered everywhere. Consequence of HID in a reflector designed for halogen. Third was a drop-in LED. Again, the reflector had to be removed to fit the bulb. This was only good for color matching the 6000K D1S low beam bulbs and the switchbacks in the turn signal. No appreciable output otherwise.
Fourth is what I have now. Morimoto Matchbox bi-xenon projectors. For those not aware, bi-xenon refers to the projector having an actuator system (a solenoid) that pulls the shield out of the way to increase the light hitting the road, usually for high-beam. I have the solenoid controlling the shields in my Matchboxes connected to nothing, as when the high beam is used power is cut to the foglights, and the ballasts are only powered when the OEM foglight switch tells the system to power on, as they get power from the factory harness. They are 35w ballasts, so there isn't an issue with using the factory wiring. Connecting the shields would have them actuating without there being light, which is pointless. It's also better to not have them down when the lights are powered anyway, since dropping them increases light above the cutoff, which you don't want when using in fog. I have them out of the car right now so I can fabricate shrouds, but when they're reinstalled I will have the cutoff sitting low, which is optimal for this application. Installation required modification of the back of the reflector. I had to grind down the original bulb-holder to push the projector forward, and I had to buy close-out section bulbs with the AMP connectors soldered onto the bases (current bulbs have connectors that make them longer). I had to do that to have the clearance needed, otherwise the back of the bulbs crashed into the battery box on the passenger side and washer bottle on the drivers side.
The output is very nice and consistent, as they are projectors instead of reflectors. Installation of projectors into the factory 300M foglight is a bit involved, as anyone who has done it or is doing can attest to. I've done it, Keith built a set to put into his car this year, and I'm aware of Robert A and Robert W doing it as well soon. It requires not only dissection of the housing (not hard at all, unscrew the adjuster as far as it will go, use pliers to gently pull the reflector from its swivel mount, then unscrew the adjuster the rest of the way, easy squeezy lemon peesy). Cost is another factor. If you can find a 2" projector used, it can be inexpensive. I found that pretty hard to do, so I bought the Morimoto Matchbox kit from The Retrofit Source. The standard kit is $270. I got it on sale during their Black Friday sale, I think 20% off since that's all I bought in the 2014 sale. In the 2015 sale I bought a lot more stuff and got 25% off of what I bought then. Doing that had the benefit of a solid projector system for what it is, along with the best aftermarket HID system money can buy (the crap on eBay or Amazon, or other resellers that you can get for $50 is worthless).
Once I have the shrouds done I think I'll leave it alone for a couple of years, but I've also got the Morimoto XBLED Universal in mind. It should fit well in the stock reflector and require far less fabrication. If that was available when I bought the Matchbox, I might have gotten that instead. I've yet to look at the dimensions for depth though. Height and depth are the major considerations when fitting something to the reflector and getting it to fit into the housing.