I have tested this on a DUO-R system and this should be the same for the regular DUO and DUO-RX and possibly true for the PC-Engine + Super CD-ROM attachment. I tested by trying to load a Super CD-ROM game (Lords of Thunder). It loaded as expected and played fine with System Card 3.0 loaded and gave the proper error message when trying to load with the System Card 1.0 loaded.
You can load System BIOS images Version 1.X up to 3.X and play CD-ROM and Super CD-ROM games. This mainly would come in handy for the Altered Beast CD-ROM which I've read only works properly on System Card 1.0. Or to avoid removing and inserting the Turbo ED often. To play Arcade CD-ROM games you will still need to remove the Turbo ED and insert an Arcade Card DUO. Loading CD BIOS images might come in handy for someone that is missing their system card but only for regular CD-ROM games as Super CD-ROM games require the extra RAM in the system card. The Super CD-ROM has the System Card with RAM built in.
If anyone has any other hardware setups please test and post the results so this thread can be used for reference. We still need someone to test CD BIOS loading with the following setups.
Turbografx 16 + CD-ROM Attachment
PC-Engine (any model) + CD-ROM Attachment
PC-Engine (any model) + Super CD-ROM Attachment
DUO-R works with System Card ROMs 1.0 to 3.0
DUO and DUO-RX should be the same as DUO-R