After many hours, I have advanced scrolling working in ProSonic. By advanced, I mean it doesn't just take the background and move it across the X axis at a given speed. It scrolls vertically too, and it can simulate layers. This means that designers will be able to split the background into sections and have each section scroll at a different speed both horizontally and vertically. Most levels in the original Sonic games didn't take advantage of moving different layers at different speeds vertically. It wasn't until Sonic the Hedgehog 3 that this was done (the water in Hydrocity and Launch Base). My engine fully supports it.
Obviously the background looks messed up in the picture, but it's supposed to demonstrate how the sky can be split into seperate sections that scroll at different speeds. Each layer will be able to have it's own filter to use too. This is all done on a single plane which means it won't slow the engine down much at all.