Here's a new thread I'm starting to share new things I've learned digging through the game.
First new find, at least to me, fielders throwing the ball get penalized on a gradual basis. It's not binary fast/regular throw.
The direction and A are pressed quickly together, and the game measures the gap between them.
Same frame: +128 power, so the throw is 640 instead of 512.
1 frame apart: +96.
2 frames apart: +64.
3 frames apart: +32.
4 or more frames apart, or direction held first: no bonus, so it's the plain throw.
You really do have to tap both almost at once. A slower press gives the normal throw.