I just returned from a fastpack trip with an ultra-runner. We were all off trail though, but doesn't really matter, except that a compact shelter is key in that situation.
Anyway, I use
Shelter: Half-pyramid type full enclosure and skip the bivy. Full protection, good ventilation and easy to set up. Can be set up with just one poll (excluding SoloMid) or hung from a tree etc. So that would be an MLD SoloMid, Oware Alphamid, Gatewood Cape or Zpacks Hexamid with Beak. A Six Moon Designs Lunar Solo will be much warmer, but is heavier and if you don't set it up right you'll have condensation issues if the weather is right for it.
Pad: 1/8" full length closed cell foam, then a short inflatable like an Xlite or NeoAir on top. The foam pad insulates and protects the inflatable from punctures.
Ground Sheet: PolyCryo, Cuben or Tyvek. I use a PolyCryo or our LiteTrail one (prototype) because they are lighter, but a bathtub cuben would be nice.
Sleeping Bag: I plan for temps in the low 20s, and use a Katabatic Alsek bag supplemented with my extra clothing (polartec tights, Cap4 Hoody, UL Down Montbell Parka). 12 oz down in bag minimum is what I bring to Sierras 3 season. Without the bivy, I've never had any serious down wetting issues from my own body moisture using a well-ventilated shelter.
This is all ultralight and light enough for any speed trip, but has a high degree of comfort.
It's such a personal thing that we could all use something totally different and still do the same trip at the same speed and at the same relative comfort/effort level.