JXQ, holding down doesn't make a difference. In order to spit the shell and catch it in the air, release the B button a split second before you spit the shell and it isn't too hard to time it so that Yoshi can catch it immediately after losing a bit of altitude. Just make sure you're holding left or right as well so you won't lose the shell in the horizontal direction. I tested and this is faster than the pit stop taken in your published movie. I don't know if the vine would be faster though.
edit: Hey guess what? I think I found an actual timesaver with the Yoshi flight trick! First off, there's a much easier way to execute the trick than the other five methods. Simply fly up against a ceiling and press the A button to bounce off and on Yoshi in an instant. As for the actual TAS timesaver, in Pipe World 5, you have an annoying delay where Mario and red Yoshi must wait for those alternating piranha plants, right? Well, when Yoshi is flying he also has the property that if he hits an enemy (even these special plants that can kill Mario while he's riding Yoshi) he will just flicker and become invincible for a short period. If you run and stealthily exploit this a few times under those pipes you ought to be able to get past that part with much less delay, if you can pull it off!
edit: Check this out. Another Yoshi trick that could save time in special situations:
This first screenshot shows an alternate faster path (at the cost of losing the message block) but it isn't a Yoshi trick since Mario could do it alone. It's faster because with Mario already in the block he'll have to backtrack less after he picks up the key around the corner.
(Ignore the numbers on the timers since I clearly didn't take these screenshots from a sequential movie.) However, at the next corner Yoshi can save Mario the couple seconds it takes to swim around to under the key. On top of that, Yoshi retains Mario's swim-with-an-item power so he can come along to the end of the level. In this particular instance this Yoshi trick doesn't save that much time, if any, but these sorts of tricks are something to consider.
-hanzou
