This add-on brings Knuckles to Sonic CD and Amy to all games. This is new and a good addition! Anything else is barely noticeable and because of that, the addon is clearly overpriced. That's a fact.
Another fact is that SEGA showed the middle finger to everyone who preordered the "premium" version of the base game by bundling the "exclusive" contents with this update. The preorder should have included this update for free, but no. This was the last time for me. And as a little extra, you got to be online for playing this game. Ridiculous!
To complete all of this, SEGA has also removed the "classic" version of Sonic 1 to 3 from the Steam store before releasing the remastered compilation INCLUDING the already remastered version of Sonic CD. You kept them if you purchased them before, but still.
Anyway, go on and grind every penny out of your fans for >30 year old games.