"The ultimate collection" is a CD out on Hip-O Records (a Universal division) and it does have three Traffic recordings but it has a lot from "Alone together", one from DM & CE, the rare "single only" track "Satin red and black velvet woman", three from "Headkeeper", one from "It's like you never left", two from "Let it flow" (including WJD and it sounds the right speed to me on here!) and the title track to his "Two hearts" album. Although some choices could've been better chosen (I think), a good overview of DM's career.
You mentioned you were interested in "early solo stuff" of his. There is a UK vinyl album (out of print now) that you may want to consider tracking down called "Scrapbook" on Island Records which is a double record set and the first album is Traffic recordings but LP one does have the single only solo track "Little woman" and an unreleased otherwise take of "Feelin' alright?" and LP two concentrates mainly on "Alone together" & "Headkeeper" stuff (though the "Sad and deep as you" they chose is from Traffic's "Welcome to the canteen" album). Once again, not authoritive, but these are pretty good collections for early solo stuff ("Scrapbook") and a career overview ("Ultimate collection"). Hope that helps.
John