FFIX's slowness is mostly about long battle intros and the way turn actions get queued up too far ahead (paradoxically, keeping the ATB running during long animations instead of pausing it a la 6/7/8/X-2/XII makes the battle system feel *slower*). Not sure simply speeding up the ATB bar will fix that.
It's a full-on everything speed-up toggle, like in an emulator, so it should circumvent the issue entirely. Probably not the best way to play the hardest bosses for obvious reasons, but at the very least you can use it to skip the actual animations.
Oh, I could have sworn it was. Maybe I'm thinking of VIII?
Yeah. FFVII Steam was more of a "Oh OK so you people are too fucking good for our service with DRM well have it your way" iso dump. With FFVIII they started adding these new features.