Yes, if you have an ISP programmer and 6-pin header.
The firmware changes mentioned above do not affect performance. In production testing, we found that 2% of C5's could not boot properly due to [exceptionally] above average capacitor values on the supply rails. Longer boot delay enables proper booting for every C5 we've tested to date.
Summary: Firmware upgrade is not necessary if you already own a C5. Every unit is tested.
Thanks for that info!
Relieved to hear the upgrade is not necessary. Doubt I'd have the technical know-how to do it anyway.