I'll profess ignorance, but is there a difference between rendering a game at a particular resolution and displaying it a one lower (as Sonic 2 supposedly did) as opposed to rendering at a higher resolution AND displaying it at that resolution?
If so, then the comparison between Sonic 2 and the SNES's hi-res mode may not be entirely comparable.
Well, here's an example of one SNES game that used the SNES's high resolution mode, RPM Racing
This was probably the first SNES game to use 512×448 on the SNES. The BG layer is running at 512×448, but as you can see with the dithering, it is also limited to a pallet of 16 colours total. The foreground layer with the vehicles is running on the standard SNES resolution of 256 × 224 using a higher colour mode. The HUD resolution is also the same as the vehicle layer.
Now here is RPM's sequel Rock N' Roll Racing:
RnR Racing runs entirely at 256 × 224 (but I stretched this image out to 512×448 myself) The backgrounds are lower resolution than RPM's but are a lot more colorful. The vehicles are at the same resolution for both games.
Other than having some smother looking background environments, the tradeoff isn't really that great at all for RPM Racing. If they stuck the vehicle layer onto the same resolution ad the BG, it would be limited to that same 16 colour palette as well. The game would look like a high resolution NES game on steroids.
Sonic 2 two player mode doesn't mix and match resolutions like this on different layers, it just flat out runs the game twice vertically at 480x320.