Normally you can control that, so on mine it works, I went down from 10 volts to 6 or 7 volts. But you should do that depending on the difference between [Vorlauf] and [Rücklauf].
And what for? Just for fun? It won't run at full load during normal operation anyway. I thought this was about properly adjusting the heating and not a quarter-mile race.
Oh, and reducing the Hup doesn't save any electricity? And increasing the cycle runtime doesn't either? If the spread is too high after the cycle, you can lower the Hup without any qualms. Modulating heat pumps do nothing else.