They had some projects on the burner, Sega has always built things that they never release. I don't know about anything designed to add power, though.
Adding more main memory would probably be more useful for homebrew, and yes you could do it. But it'd be a monumental PITA, so practically nobody would do it even with a detailed guide. Most people don't even mod their DCs, let alone rip it apart, solder in extra memory, and modify the BIOS. So coders probably wouldn't care. Overclocking could give you a more immediate benefit for all existing software, but again not many people are going to do it.