Well the i7-980 chip arrived today, obviously secondhand but unscratched and wrapped in bubblewrap with a small sachet of thermal paste.
After an abortive attempt at installing it with no boot, post or bios comming up at all I realised that I needed to update the BIOS to a more current version.
Once that was done (had to put the old i7-920 chip back in) I was able to install the updated cvhip with no problems at all.
The old i7-920 the benchmark rendered in 13m 14s.
The new i7-980 the benchmark renderd in 8m 41s.
Not too shabby and I think there may be some room for tweaking ...