With LandRun coming up I am starting to take a more serious approach to this as well. Some items I can review; others I literally just received so I don't have any real thoughts on yet. Everything I purchased was researched to some degree as to what would actually work for my needs.
Hat: Got a Gore-Tex thermal cycling cap from Swrve. Not sure if they make it anymore, but it has proven itself many times over.
Jacket: Gore-Tex 1985 Shakedry direct from Gore-Tex. Very pricey, but recommended by Nick Legan. This jacket is very well outfitted (fit, cuffs, collar, etc) and I am very happy with it. My previous jacket was the Twin Six rain coat, which worked well, but it didn't have the insulation quality that I was looking for to combat against a cold, constant rain.
Gloves: Aerostitch Gauntlet. These were also suggested by Nick, and while I should've sized down (bought the Large instead of Medium), they have worked great as well. They are the "lobster" type with two fingers grouped together, but other than that I cannot complain. I got some neoprene ones as a backup from a local outdoors store, I will try them out as well (they pass the faucet test).
Pants: I have not tested any of these yet, but I recently purchased the Pactimo Storm+ 12hr bibs and leg warmers, Castelli Polare 2 bibtight, and Showers Pass Transit pant. After trying everything on I feel like they they would be best suited for wet-wetter-wettest in that order. I can't imagine riding the entirety of LandRun in the Transit Pants, but if it's 38 and raining the whole way it might be the only way to survive.
Feet: I did not want to buy any special rain or winter specific shoes or boots, so I decided to layer on both sides of my current shoes. I got the Showers Pass waterproof socks, and while they are fairly bulky (not a normal cycling sock fit, even compared to heavier wool thermal socks) they fit in my shoes with some effort. I also got a set of Endura mtn overshoes, which are mostly neoprene and very sturdy. Nick suggested the Velotoze rubber overshoes, which are very nice, but I don't think they'd survive 9 miles of walking in Oklahoma.
With all of that said; in the most extreme circumstances I now have gear that would last a 100mi cold deluge. It won't be comfortable (it never is), and it wasn't cheap, but if it means the difference between a finish and a DNF it's worth it. More to come if I get some rain miles in the following weeks.