I have DotEmu's version on Android, and I see nothing 'remastered' beyond my copy of the 15th Anniversary on PC. My (CD) copy of the 15th Anniversary one also has the development diary (printed) and the making of video, so that's nothing new. I assume they were already on GOG before the 20th Anniversary edition went up?
Can't really tell with the sound as I can't be arsed to fire up WinUAE to run the ADF on the CD, although the 15th Anniversary one already sounded fairly crisp from memory (with the exception of the voices, where you could hear some noise in the background).
The 15th Anniversary edition has two difficulty levels, as opposed to the 20th Anniversary edition's three. The original 15th Ann. difficulty levels were "Easy" and "Normal" (the original 90s version had none), so I'm not sure what this third level is (whether it be ultra easy or hard, I suspect the latter).
So basically the update is only the extra difficulty level and, if you're buying on Steam and are so inclined, the achievements.
But still, good to see a classic title still getting the love, and good that the "Steam or no sale"-ites get a chance to play it.