I don't understand, when I lower voltage, my I/s falls from 168 to 147 and Sol/s falls from 315 to 275, but my Shares per second, for what I receive payment, don't change. Does it mean that difficulty of share also vary and I will get less money or not? Is hashrate more valuable than shares per second?
If difficulty can go lower to accommodate your lower hash rate then it will do so. There is a limit of how low a difficulty can go, set by the pool. But yes hash rate is more important as youre solving bigger pieces.