Hey OP do you have a kill-a-watt you could purchase locally. If you are in the states Home Depot and Lowes carry them. If you can find one locally I am sure we could get together the 3 or 4 BTC to get some accurate power readings.
The kill-a-watt brand doesn't appear to be commercialized here in europe, and I've been searching for an equivalent device locally each time I've had a chance to head out to a store for the past couple of days, but no luck so far.
I also took a stab at modifying DiabloMiner and managed to get it to use 16component vectors, which is what GCN is supposed to be tuned for, but performance isn't what I expect and its really hard to profile/debug the tahiti since I could not find any development tools that specificly support it yet.
BTW, they do make 240v/50hz euro Killawatts, but you might have to order it from the US. They also make 240v/60hz (double hot, like ovens and water heaters) ones and 208v ones for DC shit. Might have to look around, I love mine, its been essential for planning stuff out.