Megadrive: 8MHz 68000 and game-oriented graphics chip (tile based in particular, all pre-defined, means little-to-no CPU effort in drawing new graphics on the screen)
A500: 7.14MHz 68000 and planar-bitmap graphics
Not only is the CPU a little slower on the Amiga side, but to try to do all the Megadrive effects would be counter-productive on top of that.
However you can cut out some Megadrive features, and add in Amiga capabilities to compensate. Decent overscan, for example.
What makes Sonic is the really fast scrolling sections. If you can pull that off on the A500, in 32 colours, then you have something to build upon. That probably means being able to scroll the screen by up to 64 pixels per frame in any direction (including the drawing of the new section of the map) in addition to animating the spinning rings, sprites, etc.
In addition, I'd be a bit dubious about ripping the Sonic graphics, as the franchise is still very active.