We have developed a fully ab inito workflow for predicting ZT , by combining state-of-the-art approaches to computing the S, σ , κ el and κ latt , which we and others have validated against multiple classes of TE materials including chalcogenides, oxides and oxychalcogenides. In this study, we apply this workflow to obtain high-quality reference predictions of the thermoelectric performance of π SnS and SnSe. π SnS and SnSe have lower κ latt and higher (absolute) S than the corresponding orthorhombic phases, but at the cost of higher carrier effective masses, stronger electron scattering, and lower σ that requires heavier doping to offset. The majority of the κ latt occurs through intraband tunnelling characteristic of amorphous materials, leading to very low thermal conductivity at and above room temperature. With n-type doping at the level achieved in experiments on p-type SnS and SnSe, π SnSe has a lowtemperature ZT comparable to Bi 2 Te 3 and a high-T ZT competitive with orthorhombic SnSe, resulting in a large average ZT for low-to mid-temperature applications. More generally, these findings demonstrate targeting structural complexity as a strategy for suppressing the low-T κ latt in TE materials to optimise the ZT toward more widespread heat-recovery applications.