Managed to undervolt without flashing the BIOS. Just needed to enable the core voltage option in MSI's Afterburner software and that's it!
At the moment I'm running overclocked getting 670MH/s @ 1125/975 AND undervolted from 1175mV to 1112mV, for a 37W reduction from stock settings down to 230W power draw for the card at this speed. The fan is also down to a more bearable 50%
So with these stable settings the efficiency figure is 2.91 MH/J. This is smack in the middle of my estimated figures from earlier. I'm sure that lower core frequencies will allow for even bigger undervoltages.
The funny thing is that the card would actually run stable at 1040mV with a 197W draw as long as the only thing it was doing was mining which is 3.4MH/J. But fire up a game and it would crash after a short while. Maybe this is due to the power management features turning things on and off as was mentioned earlier in this thread? Anyway, it seems that head-less mining boxes using these cards have room for more efficiency gains.
Tomorrow I'll measure some more data points to get a better idea of what these characteristic curves look like.
P.S. I dunno if its a bargain for mining or not but I'm having a lot of fun with this card so far and mining quite a few bitcoins in the process