Voltage has no direct correlation with performance. A module rated for a speed at a voltage can run at a higher speed with a higher voltage, until other factors become more important. Other factors can be a limit on how much voltage the motherboard can apply or the RAM modules cooling etc.
A 1.25v 1600MHz module with 9-9-9-24 timings will perform identically to a 1.65v 1600MHz module with 9-9-9-24 timings, assuming no other variables are different.
However, that same 1.25v module being run at 1.35v or higher will outperform the module rated at 1.65v because it has greater performance per watt.
Does that clear up any remaining confusion?