I've been on UPZs for about 6 years. I really liked a pair of red 26M RSVs that I had, but they were a little too big. I downsized to a 25M RC10 this year for a tighter fit. I really like the boot and the super precision that the stock liner gives (zero heel lift, zero foot slop, much more control than before), but I can't stand that liner for more than 2 runs before I have to get out of the boots. So far, I've tried to make 3 liners work without success: the stock liners, new 25M red Speed liners, and my old 26M black Speed liners.
Noticing the stock liner discomfort when I bought the boots, I ordered a new pair of 25M red Speed liners, and took them straight to a bootfitter to heat mold before the season began. He said they are too high volume for the boot (they bulge from the shell somewhat), and didn't even try to heat mold them. He said to stick with the stock "racing" liners, so I took his word for it and put the new Speeds back in the box. At the time, the worst part about the stock liners was pain around my ankles, so he ground out some of the shell. (That hadn't even been an issue when I "rug" tested the boots with my old 26M black Speed liners.) He also made custom footbeds for me.
Now that the season has started, I've tried the stock liners on snow twice and have crazy pain in my arches, and something is up with my big toe that feels like it's sprained in that liner. Each time, I had to stop and throw my old 26M black Speed liners in to get through the day, which feels sloppy, but eliminates the pain. I need to do something, and I think the black Speed liners (even if got the proper size) are just too soft. I've love the have the stiffer red Speeds in there, but was told their volume would be an issue. Has anyone else used the red Speeds in UPZs?