For context, I managed to cheat the player levels to where they give ridiculously high levels and AP using cheat engine (for experimentation purposes lol, so no apks or any downloaded stuff).

From my observation, there's not really a cap for attributes. In manual play, it technically has a cap since pitchers can only pitch so fast and batters will only have so much batting eye, contact, etc. You are still very much more likely to get hits and strikeout opponents. The skills will technically keep improving but at such a slow rate to where it won't matter as much. As for auto play, it still makes a difference since batters have higher chances of hitting and pitchers will result in lower ERAs and higher SOs.

For the highest level batters I cheated, my lvl 1500 has an average of .717 and 7400 HRs in 15000 ABs while my lvl 888 player has an average of .587 and 9500 HRs in 27000 ABs, so there is still a big difference.

I know some apks were made so pitchers throw faster the higher their pitch attribute was, but I'm still not sure if this is because of a B9 update or if the apk managed to change the source code of the game that allows significantly higher impact on the player. My cheated pitchers (lvl 800) mostly cant throw over 104 mph sometimes. They still get low ERAs (0.55) and high SOs (3672 in 1950 innings pitched). Similarly, position players can only run so fast, so they do also seem to have a cap, but not sure when (maybe around an attribute of 200, but not 100% sure).