This work investigates the influence of cavity mirror properties on the transmission bandwidth and detection sensitivity in cavity-enhanced dual-comb spectroscopy (CE-DCS). A theoretical model was first established to describe the effects of mirror dispersion and finesse on the transmission bandwidth and the noise-equivalent absorption (NEA), based on which low-dispersion, high-finesse mirrors were designed and fabricated. Without spectral extension, the system achieves a 370 cm-1 transmission bandwidth (~ 88 % of the original comb) with NEA = 1.45×10-10 cm-1Hz-1. With EDFA-based comb extension, the bandwidth increases to ~ 400 cm-1 and the NEA is 2.01×10-10 cm-1Hz-1. These results demonstrate broadband and highly sensitive gas-absorption measurements with the proposed mirror design, enabling precise trace-gas analysis.