Synchrophasors are an emerging power system wide-area dynamic performance monitoring and control technology. The IEEE standard C37.118.1 outlines specifications for the construction of synchrophasor units, in which two reference synchrophasor models are proposed for protection (P-type) and measurement (M-type) purposes. This paper develops improved synchrophasor models for power system dynamic stability/performance evaluation based on the reference architecture proposed in the IEEE standard C37.118.1, considering frequency error (FE) and the total vector error (TVE) as the main performance indices. The quadrature demodulation filters are designed explicitly considering the impact of harmonic distortion and out-of-band interference, and thus optimized quadrature demodulation filters improve estimation performance during adverse conditions. Furthermore, a special emphasis is placed on the input low-pass filter to attenuate high-frequency components above the Nyquist rate. The improved synchrophasor models are rigorously tested to ensure compliance with performance requirements stipulated in the IEEE standard. Results show that improved synchrophasor models significantly enhance the FE while ensuring compliance requirements for the TVE.