I know of at least two cases where GH1's that used the hacked firmware sustained permanent damage to the video processor "Venus Engine". In both cases the owner was told the repair would cost ~ $800. Apparently the damage is easily identified. If you look at the GH2, it uses two totally different processors that can handle the increased sampling rate. If improving the performance was as simple as a firmware hack, I think that would have been much easier for Panasonic.
I also saw a GH1 on EBAY, being sold "as is", a victim of the firmware hack gone wrong.