I red this post today for the first time...
3-4 months ago i swapped my GD rom Unit (Samsung 3.3 V on plate) with a Japanese Yamaha Unit (nothing on plate about voltage).
It is working flawlessy
I have a PAL v1 DC.
I would like to add that they are not perctly mechanically interchangeable though (you need to make an extra hole on the plate to fit better on V1 dreamcasts)