Voltage has nothing to do with the workload, just the hardware's stability. In general, the higher the voltage at a given frequency, the more stable it'll be (until heat or junction breakdowns become an issue).

The amount of work done is proportional to the operating frequency. I see about WU 1.53/MHz with a 2-chip stick, so 225MHz should run about 344 - plus or minus, because USB traffic and pool latency and whatever.