Not totally, but there is documentation out there on the SGX hardware. I know the SGX has two 16-bit HuC6270A VDCs, where the rest of the -Grafx family has only one. Also, there is a unique chip, the HuC6202 that allows the two VDCs to work cooperatively. The rest of the consoles don't need this as they have only one VDC.
The SGX also has 32KB of work RAM, where the Cores have only 8KB.
That's it in a nutshell, but
here's a document that goes into detail on the differences.While it's probably possible to mod a Core or Duo to become a SuperGrafx, the real problem is the HuC6202 IC. This was a proprietary chip whose only known use in history was in the SuperGrafx. You see where I'm going with this.... You'd have to butcher a real SuperGrafx to obtain one of these to throw in any sort of modded Core. Sort of defeats the purpose.