So, I am trying to just "clean up" this tough piece that I've worked on, shelved, and worked on (on and off) for almost a year now, and bowed of 2 recitals because of it (but that's a different story). Anyway, I'm trying to give it another go, and clean up a few problem spots.

My body has been used to playing this at tempo for a few months, and now when I slow it down, it felt even harder and I had some control and timing problems.

My question is...in theory, if you've really mastered the piece, you should be able to play it at 10% or 100%, and have no issues dialing it back down - right? I'd like to hear your thoughts on this.

Slower than what you're used to is very hard. At least if you're sensitive to playing in time. You have to use so much more control to make it sound good.

This is not the same as playing short phrases slowly to master some movement. To actually play the whole piece/tune in a much slower tempo is as difficult as playing it in a much faster tempo.

When you get really slow it becomes quite difficult to play exactly in time with a metronome as the gaps between the beats are so large. But it is excellent rhythmic training so well worth the effort I think.
I used to do a lot of practice with really really slow beats and it has been great for my sense of rhythm.

My teachers have often recommended slow practice even when a piece has been almost mastered at tempo. Besides focusing on careful timing and phrasing, very slow practice is also a great way to restore a sense of mindfulness about the piece. Sometimes your concentration can wander as you repetitively lock the piece into an unvarying performance routine. Drastically changing the tempo (or other variations on your practice) demands that you put everything about the piece back into the center of your consciousness, and it can help develop a habit of engaged mindfulness for the final performance

I couldn't agree more with packa. One of the hardest songs I know used to be painfully difficult at a slow tempo simply because I was conditioned to playing it at the "proper" tempo. I've discovered some very pleasant sections that I like playing slower from time to time.

I'd also say slow practice is good for helping memorize a song too. I know it helped me. The first time I tried playing the piece mentioned above slowly was very hard even though it was already memorized. I actually had to play faster to know which keys to hit next when I was starting out. Now I am more easily able to play the song at an extremely slow tempo because of that practice.

I had this problem with one piece I mastered at always the same speed. It was very difficult to slow down. I had to lay down the piece for a couple of months, before I was able to play the whole piece at a slow tempo.

After that I practise my pieces never at the same speed from the beginning. Very slow, slow, fast, faster, very slow, dead slow, fastest, a little bit slower than that, but never the same. I never had any problems changing back to a slow tempo.

Except when adrenaline is rushing through my body. Then I play always faster than I would like, even if I try slower tempo.