Is there a way to lower the power limit below the minimal 50% on Nvidia cards?
Not through software man.
Either way i advice against it since around the -30%, the speed slowdown counters any gains from the power efficiency.
There are some 1060 cards with power limit way above what it should be and even setting 50% doesn't allow to get anything below 1.0 volts

I then used the temp limit to operate them at 750mV and around 1500 MHz... This kept the speed same in ETH especially on micron chips those cards cant go beyond 23 MHs so they dont need any core power at all.
Wonderif we can trick the card somehow to use low voltage