Sonic 2 Hacking Guide
Genecyst Savestate - Advanced Configurations

Advanced configurations includes things that don't nessisarily benifit the player in general, but change other things instead.

011A78 (F600) - Screen mode

00 - Sega logo
04 - Intro
08 - Sega logo (slow fade)
0C - Game
10 - Special stage
14 - Continue
18 - 2P results
1C - 2P level select
20 - Ending sequence
24 - Options
28 - Level select
88 - Restart level in timed mode (used for demos)
8C - Restart level (used when killed)

011A7C (F604) - Buttons pressed
011A7D (F605) - Buttons pressed
011A7E (F606) - Buttons pressed
011A7F (F607) - Buttons pressed

For keys pressed, you simply need to know the Motorolla key format:

1 2 3 4 5 6 7 8 (this is a byte represented in bits)
S A C B U D L R (these are the bits translated to show what each key is)

With a 1, that means the button is being used (on) and with a 0, the button isn't being used.

011A8C (F614) - Screen mode change timer
011AC0 (F648) - Water pre-set height (16-bit)
011AC2 (F64A) - Water master height (16-bit)
01245B (FFE3) - Sound #1 to play (sound test selection + 80h)
01245C (FFE4) - Sound #2 to play (sound test selection + 80h)
012468 (FFF0) - Demo mode (01 to enable)