Relative S/N of an atmospheric signal for all sub-Neptune exoplanet candidates (i.e., 3 M_⊕ < M_p < 20 M_⊕) orbiting M stars (i.e., T_eff < 3850 K). The atmospheric characterization S/N is normalized to G 9–40 b, which is highlighted with the filled colored symbol. G 9–40 b is among the most promising exoplanets in this sample for atmospheric characterization, particularly among those with the coolest T_eq.