The more I see such a divergent range of experiences the more I am convinced it is the hardware that is to blame for problems because the firmware is a constant.
I think it is a little more complex than that, as for basically all reported problems there are additional variables people might not even think about. For example, when it comes to WIFI stability, there are different types of access points, encryption methods, connection speeds; even using a special character in the SSID might be the reason why it works for one user, but not for another.
GPS functionality might depend on your location (inside/outside, Europe/US/Australia/...), weather conditions, interference with radar or radio, "garbage" left in the settings if you did not wipe your tablet after upgrading from Android 3.2 to 4.0, etc.
There are certainly differences for the hardware revisions, but unless somebody reports that for two different Archos G9 tablets, running the same firmware at the same location and after a wipe, one would get a GPS fix within a minute and the other does not within an hour (or one would connect to a specific WIFI access point and the other would not), I don't think this is likely to be the reason for the observed problems.