Thanks for posting your dumps.
Looking at your IORegistry dump I see it contains only a 64K image (0x10000) which is what DarwinDumper saved out. You can see for yourself by loading up IOReg.txt and searching for ATY,bin_image
I'm sure there's a simple explanation as to why you do not see the full 128K ROM in OS X's IOReg but I don't know it. Maybe somebody else will be able to throw some light on the subject?
If you still want to dump the card's ROM then I understand you can save the card’s ROM from Windows using ATIFlash with a command like this (though check the command first just to make sure it is actually the right command):
atiflash -s 0 myrom.rom 20000