While I'm not going to help with your warez related questions (see the site rules), I can a few things in general about using the SD Card adapters (including this internal one) on a Dreamcast.
First of all, the SD Card is done off of the Serial port. The max speed of the serial port is lower than that of the GD-Rom drive, not to mention the extra code that has to be run in order to access the SD Card. Thus, its slower to run things off of the SD Card than the disc drive (this is for the most part not a problem with homebrew).
I see no reason why this particular mod would disable the GD-Rom drive, as you still need it to boot the machine, so I'd be pretty sure that it still works.