I planned to have a look on that too. After a quick test, it works for the pokémon entrances but it makes this too:
The black things (for the opening screen) aren't fast enough and get stuck here during the battle. Trying to check this.
(about the wrong position for battler sprites, it's normal since I upgraded from v14 to v15 and I didn't changed the gif sprites positions)
Edit : it breaks the text position too.
I got a way to fix the black thing, but only for the bottom one. "@viewport.rect.height+=8" the 8 should be changed to the new speed.
Edit2: ok fixed the thing. So to change the speed, in PokeBattle_ActualScene (search for appearspeed with ctrl+f):
The values are dependent of the appearspeed so if you want another speed than 12 you'll have to make tests. I tested and it worked for me, texts are good and I checked with small/medium/large screen options.
It also affects the trainer sprite entrance.