The use of lead-free piezoelectric materials in high-power applications requires a high mechanical quality factor Qm and temperature-stable ferroelectric properties. In this article, the influence of Mg-doping on the 0.94Na1/2Bi1/2TiO3-0.06BaTiO3 system is analysed with focus on the role of defects related to ferroelectric hardening. Temperature stability (depolarization temperature), electromechanical properties (piezoelectric activity, Qm), electrical properties (conductivity) and crystal structure for compositions were quantified. Compositions with similar amount of Zn-doping were analyzed for reference. A drastic increase in electrical conductivity at −0.3/−0.5 mol% Mg was associated with a concomitant increase in Qm. Similar behavior in Zn-doped compositions provides a basis for a more comprehensive mechanistic understanding of acceptor doping in these lead-free piezoceramics. The very high and almost vibration-velocity-independent Qm above 800 makes Mg-doped 0.94Na1/2Bi1/2TiO3-0.06BaTiO3 an excellent candidate for high-power application.