I have had one of the PowerExpress prototypes for 4 or 5 years, so I thought I would provide a link to my page about it:
http://www.isellmacs.com/macsystems.htm
http://www.isellmacs.com/macsystems.htm
Where are you getting the correct checksum with which to compare? Are you using the first group of bytes of the file or something? I do not believe that the first bytes of the flash dump are a checksum because a startup sound is located there.None of these dumps seem have valid checksums.