The Switched-Capacitor Power Amplifier (SCPA) has become a key enabler for modern wireless communication because of its high efficiency, high linearity, and high integrability. This paper discusses the impact of the extended Forward Body-Biasing (FBB) feature in 28 nm FD-SOI technology on Ultra-Low Voltage (ULV) SCPA. A new model of the Drain Efficiency (DE) and System Efficiency (SE) including body-biasing and drivers power consumption is introduced and validated with SpectreRF simulations. FBB on the SCPA improves by up to 14 % and 67 % the SE and transistors area, respectively, compared to a nominally body-biased SCPA under 0.5 V supply voltage at 2.4 GHz, while improving linearity and enhancing PVT variations.