I've got the whole set too, and I've been using the UF as a final finisher. Joel rates the UF as closer to 14,000K IIRC, and from what I've seen under the microscope, I would agree. It finishes finer and cleaner than my vintage Thurry or Nani 12k.
Don't be too quick to discard the M or F, that medium is a wicked fast cutter, great for setting bevels and cleaning up an edge, and even though I haven't used the F that much, it's scratch pattern is incredibly regular and fine. It leaves a good base to play around with other stones on. IMHO, I would rate it at about 6K range, but its real strength is in the consistancy and regularity of its scratch pattern.
Also, as you say, some blades like the Spydee's more than other stones.
I've also debated the idea of selling mine to fund a J-Nat, but think they are just too good in certain situations to let 'em go.
My take on it is that I've already got 'em, better to have 'em and not need them than to need them and not have them.
"Nothing is as it seems... Neither is it any different" -Neal Stephanson