I know you said high fired, but in my experience I've found it's a lot easier to sand at the bisque stage if possible. The 220 wet-dry sandpaper works well but doesn't hold up long on the high fired porcelain—you get more mileage out of it at the bisque stage. I make a lot of unglazed porcelain beads and small sculptural forms, and my process is to use fine steel wool on bone dry greenware (if needed, wearing a mask), then after bisque firing I wet-sand any pieces that aren't as smooth as I'd like using either the diamond pads linked above, or sandpaper. After the final firing, if anything feels rough I might sand it again. A three step-sanding process is obviously a bit tedious, but for very special pieces it's worth it for the amazing buttery finish.
Also if you happen to be making small rounded forms, you could use a large rock tumbler with water and some aluminum oxide powder which is much, much easier than manually sanding them. This could be done at either the bisque or high fired stage depending on how much material you want to take away.