As predicted by a previous analysis of the lattice thermal conductivity of the IV-VI chalcogenides, the complex structure lowers the particle-like transport below the amorphous limit, and the shallow temperature dependence of the intraband tunnelling transport leads to κ latt much lower than other chalcogenides at room temperature and competitive with the κ latt along the layering axis in the orthorhombic phases at high T . The cubic structure supports larger absolute Seebeck coefficients than the Pnma phases, but larger carrier effective masses and stronger polaroptic phonon scattering lead to low conductivity, and n-type doping with carrier concentrations on the order of 4 × 10 19 , and ideally larger, are required to obtain reasonable power factors. If this is achievable, we predict that π SnSe could show ZT > 1, competitive with Bi 2 Te 3 , 9 at room temperature, and ZT as high as 3 at 800 K, matching with state-of-the-art experiments on Pnma SnSe. With a typical 300 K cold-side temperature, this strong low-temperature performance results in a much higher average ZT than we predict for p-type Pnma SnSe, making π SnSe suitable for a wide range of low-to mid-temperature applications. The predicted performance of π SnS is also considerably better than p-type Pnma SnS, such that a ZT > 1 could be achieved for mid-temperature applications if the n can be optimised.