OMG. I would have flipped if I had dropped $400+ in late 80's money on a CD attachment only to find out that it was only used to really play music, even if only for that portion of the game. I am sure it loaded other areas as you progressed but who among us knew then that CD games "streamed" the data to the internal RAM?
I would have completely felt gypped had that been me.
I remember wanting a CD-ROM attachment bad but I held off because I did not have the TG-16 yet. I got one around 1993 or so - after the Duo was released. By then the CD attachment was long gone from store shelves.
Have you tried Neutopia I and II yet? They are very, very, reminiscent of The Legend of Zelda I on NES.
Shockman is very much a Mega Man clone as well if you are in the mood for something like that.
There are a ton of exclusives on the TG-16 that make the console more than worth collecting for. Too bad Konami, holder of those titles today, is not doing anything with them.