This game seems to have a 1.0 and a 1.1 version. Is there any known differences between the two?

When you say disc do you mean PC as well or just PS1? I'll answer what I think you're asking and say no. As far as we are aware there are no differences between 1.0 and 1.1 However if you are asking about PC vs PS1 the only real difference is that on PC you can't pause buffer dialogue.

There was a PS1 Greatest Hits release, which I'm assuming is the 1.1 ISO. I don't think there's any difference.

I meant 1.0 vs 1.1, thanks for the answer and the extra details about that PC difference as well.

Actually there is a difference between v1.0 and v1.1, although it doesn’t really effect gameplay. v1.1 has some issues with looping music. In certain stages music from a different level or part of the game will start playing when the current level’s theme finishes. For Example: In Welcome to Ant Island the Bug Bar theme will start playing as soon as the level’s regular theme finishes.

TLDR; v1.1’s only difference is audio issues not present in v1.0.