I've been looking at some of the tameshigiri demos by the Korean schools of gumdo on YouTube recently. There are some truly amazing shots in there, some of which I didn't think possible before.
For example, they have a version of tsubamegaeshi, where they cut the kiriage, and before it falls, they actuall do a walkaround the target / 180 turn cut the top piece from the other side of the goza. This requires not only a very sharp sword, but more importantly the footwork/movements to get into the right position very quickly.
Eg #2 a student jumps high up in the air to kesa a tall bamboo target near the top. I'm estimating the top of the target was at least 7' up (for a person who looks about 5' 6").
Eg. #3 multiple cuts on a bamboo target moving around it, with the final suihei after the from a kneeling position (180 from where they start).
Eg. #4 a sensei cutting several kiriages moving up the goza (like tsubame) and the pieces stay in place.
Some of these really depend on having an extremely sharp sword (e.g., #1 & #4). However, most of these require a good bit of speed, synchronization, and practice.
Are these practical? I'm not sure about all of them but I can see uses:
#1 - Quite practical - it's better to be moving away and around an opponent, and if you can start the cut on one side and do another cut on the other side, you're much safer off (forces them to try to turn)
#2 - Somewhat practical - yes, there can be attacks from above, eg. someone on the eaves of a roof, or on a level above you. You can wait, move into position, or just prepare to surprise them with a high jump to attack them. The skill is in NOT landing/tripping on your sword.
#3 - Somewhat practical - Dropping from standing to cut a suihei at the knees from a kneeling position is real. Lots of ken styles have this. The skill is in moving into this position, but not getting stuck in kneeling. That is, you should be able to quickly get out of kneeling, and fight from such a low position as well.
#4 - Not practical - This makes a cool demo of skill but do you really need to cut a target three times along the same direction? They would have moved or blocked by the second cut. Okay this has got to be an extremely thin and sharp sword. He is obviously cutting the target, but there is so little movement in the goza that it seems like the sword is not the right cross-sectional shape. The cross-section of a wedge is crucial for cutting heavier, or denser objects. Otherwise the blade edge bends and twists too easily.
I'm amused by the jedi-like style here in #4, but unlike the movies, real swords take damage on hard targets, and targets usually move too.
We have a particular cut sequence that combines seven of our katas performed on one target. This requires at least 7 cuts on a 3' goza, with another optional 3 cuts and two stabs. It isn't practical to do so many cuts on a single target in a combat situation. This is more for demonstrating skill, accuracy and our katas performed on an actual target.