That whole "Power consumption scales with frequency cubed, a 20% reduction should result in around half the power consumption" is completely wrong. Power consumption is linear with frequency and squared(not cubed) with voltage. The formula is P = CV^2f, where C is capacitance, V is voltage and f is frequency.
If you figure capacitance is roughly average on a specific die during typical work, its pretty easy to figure out. Of course, as stated above, this doesn't account for leakage.
For the sake of arguement, say you had a 500mhz GPU at one volt and its power consumption was 50 watts. The lets say you can run it at 750mhz at 1.3 volts. Power consumption would be 1 * 1.3^2 * 1.5 = 2.535 times the power, ~126 watts. Again not including leakage. This isn't to say a 50% increase in clock = 2.535 times power consumption, it just shows the formula relative to a specific voltage. If you were to use frequency cubed, in this case, you'd end up with 3.375 times power consumption for the same increase, quite a bit ahead of the actual formula.
So to define, as the example talked about above, what Tahti uses at a lower clock, you need to find what voltages you can run a lower clocked Tahti at. Stock reference is 1.15 volts. Anyone with a 7970 want to test their shiny new card at underclocked/undervoltage?
Edit: Some quick brain math says that if a tenth of a volt in Tahti is gaining close to ~30% overclocks, then I would suspect dropping the core clock by a third would still require one volt. Using the formula above, if Tahti @1.15v and 925mhz = 250watts, Tahti at 1.0v at 608mhz would = 167 watts. Of couse, this is the full board power with RAM and all, so if that stayed the same(RAM amount, clocks, ect) while only the chip dropped you might be more up around 180 watts. Still nowhere in range for a console. If you cut the clocks in half and got down to 0.9v or 0.8v you might be in range for a console chip but at that point there are much better options open than using a large chip underclocked. Theres a reason no one takes a large chip and then underclocks it for lower end chips.