The only difference between the SE FDHD ROM and original SE ROM is the inclusion of the FDHD driver. I assume this is for Mini vMac, which bypasses the hardware driver altogether with its own custom driver for accessing disk images. I think Paul Pratt addresses this in the build notes somewhere. Essentially, there is absolutely no difference between them for the purposes of Mac emulation, since Mini vMac does not use ANY of the ROM drivers, because it does not access any real hardware.