Taxman version differences:
-Special Stages run at 60 fps instead of 15 fps
-Sonic 3 elemental shields can be toggled on/off, replacing normal shields in level layouts
-Knuckles present, and Tails can fly
-Time Attack Mode ala Sonic CD, Sonic Mania, etc
-expanded 2-player mode with double the levels of the original, including split-screen 2-player chemical plant zone (which was impossible on the Genesis for hardware reasons related to water effects)
-2 player mode now includes Sonic 3 elemental shields by default (can be toggled on or off)
-Multiple game saves ala Sonic 3, Sonic Mania, etc
-new 8th special stage
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2 versions of Hidden Palace Zone - both the original proto version accessable through debug mode, and the finished new version in Mystic Cave Zone
-Switch between Sonic 2 and Sonic CD style air-physics
-Boss attack mode (with new, unique art from a scrapped level based on Techno Tower Prototype from Sonic Crackers):
-720x480 16:9 resolution as opposed to 320x240 4:3 resolution of the original
-occasional objects such as shields now use true transparency
-Every single patch difference Sonic & Knuckles provides to Sonic 2 is included in the Taxman port, most of which can be explained here:
https://s3unlocked.blogspot.com/search/label/Lock-on
It is impossible to add wide screen support to Sonic the Hedgehog without rewriting enormous chunks of the game and also redefining how the Genesis hardware itself works. The reasons are too numerous to list as to why this is not feasible from a rom hack even with a custom emulator. It ties in directly to how the timing of the game itself works.
The type of work that would need to be done to make the Sonic games widescreen has already been done. In short, the process is "1) define new virtual hardware that extends the capabilities of the genesis, and 2) Rewrite the sonic engine and object definitions in game to take advantage of the new virtual hardware." That is literally why the Taxman games exist at all. His ports are already exactly that.