What was your plan for using SheepShaver to install the firmware update?
EFI is firmware-on-disk (OS X also uses EFI files to boot), so you're going to need to overwrite the existing EFI file on your synth with this one somehow. EFI uses the PE header structure like Windows executable files, but it's not an .exe. It won't "run" on any computer system, as it's the actual firmware needed for your synth; the only place it will run is when copied to the synth's EFI storage location.
This leads us back to SheepShaver: is there some reason you're attempting to load this via a PPC emulator (with minimal serial communications capabilities)?
I can't find anything in
http://www.synthfool.com/docs/Roland/Ro ... Manual.pdf that indicates its firmware is even updatable, let alone how to do so.