For stable operation, I would say don't go below 1 V (from looking at a couple of threads about 1.05)
That said. Power does not always go down when you undervolt a CPU.
Two things are fixed P=ExI and I = E/R. with a Resistor it is a given, if you decrease E then I goes down and Power goes down. Power goes in the direction of Cause - in this case cause is decrese in E. This aften is not the same with a active component such as an IC. The problem here is that quite often when you decrease the input voltage to a circuit, Current does not always go down, infact oten increses and may infact increse power. To save a couple of Watts, not worth it.
Bottom Line - The change in HSF should cure your problem, don't sweat the undervolting.