What I found while trying to do this is that I could get characters to move faster, but the camera would not keep up. I never did find the camera code.
Do the camera stop when you stop, or did it continue scrolling up to the moment you're in the center again ?
How does the camera reset itself when you step on the teleport tiles in dungeons?
and more generally, are collisions properly handled ?
For what it's worth, the Phantasy Star 2 version included in the Phantasy Star Complete Collection for PS2 has options to increase movement speed, EXP multipliers etc. The game basically runs using an officially sanctioned Sega Genesis/Megadrive emulator, so I'd wager they've got to be poking the emulated system's memory. It should be possible to get some useful info by disassembling this, though this might be a roundabout way of doing things.
It's overkill. If you really have to disassemble something, disassemble the Genesis game. But even that would be overkill, and lorenzo already did it anyway