This seems like a strange game to do any speed runs with, but there are a couple of options:
1. Endless mode (Type-A), high score. You could simply play until you reached a score of 999,999. If you start at level 19 (naturally, you should), it doesn't take all that long.
2. Type-A high level. I honestly don't know how long someone would be willing to play, but note that once you get to level 29 (I've recently discovered), the pieces fall even FASTER than before, meaning that they're falling faster than you can physically drop them on any previous level. It's HARD. Even playing at low speeds; moving the pieces left or right fast enough is not an easy task.
3. Type-B game on various levels.
On top of these options, things to take into consideration:
1. For high scores quickly, score only Tetris'; meaning, only clear 4 lines at a time.
2. If you're scoring only using Tetris', keep in mind that you can get more of the other pieces (that is, other than the straight 4-block piece, required for scoring a Tetris) dropped more quickly if you keep your plane of pieces closer to the top of the screen than to the bottom (keeping the plane by the bottom would be safer, but we're not going for safe, are we??? ;) )
I think it's interesting to watch, but it can get rather boring quite quickly. I think doing a Type-B game would be the most entertaining. I have a Type-A movie in progress of Tetris-only scoring that gets to level 29. It takes a while and gets repetetive quickly, but if you can do really well (and abuse the randomness factor), keep your plane close to the top, and move swiftly, it could be fun to watch.
I dunno; just some ideas. Anyone have any input?