I'm not sure why anyone would want the SD2SNES Pro to able to play as a SGB. The requests here have been for A) save states (which I can understand a wanting for) and B) SGB support. The SGB2 coupled with a GB X7 is a far superior solution. For one it won't be emulated or simulated and two, and IMO the best reason, No linking of game boy games. I can see someone linking two sgb2 and two GB Everdrives, but with a sd2snes wouldn't that be impossible? and negating what the Game Boy in general is all about? (This is obviously just an opinion, I know some people like solo GB games but the fact that the SD2SNES Pro wont have linked capability kind of makes a possible future implantation sort of "meh")
The biggest fault of the original SGB, besides having its timing off, was the lack of a link port.