I went true to size on my UPZ's, and they fit great right out of the box. I ended up having to upsize a few years ago, after my arches dropped and my feet got longer. There's a significant difference in the midfoot volume between the 26 shells and the 27's (similar to what Corey noted when he moved up). The 26's used to be a wrestling match to get on, even with my skinny feet. The 27's are night and day easier. If you have high arches, you'll regret sizing down.
I ran my 26.5 Black Diamond liners in the 27 boots for a couple of seasons (unlike the shells, they stretch). Try the stock liners, but try your Palaus in there too. You might like the feel better, and it will give you a comparison of the wrap liner v the tongue liner in the shells. I personally like the tongue liner better with the flex of that boot, but I seem to be about the only one.