Thermally induced decarboxylation cross-linking of carboxylic acid bearing polyimides has been recently introduced to create cross-linked polymer membranes with enhanced gas permeability. The main focus has been on the cross-linking of high permeability/low selectivity 6FDA copolymers with a relatively low amount of carboxylic acid groups. In contrast, decarboxylation cross-linking was applied in this work on a low permeability/high selectivity polymer with a large amount of –COOH groups (pure 6FDA-DABA). 6FDA-DABA membranes were thermally treated at different temperatures (100 °C, 180 °C, 250 °C, 350 °C and 400 °C and tested for CO2/CH4 and CO2/N2 separation and thoroughly characterized by ATR-FTIR, TGA-MS, DSC, EDX, XRD, density measurements, fluorescence spectroscopy and gas sorption measurements. Two counteracting mechanisms defined the overall performance of the cured membranes. Physical tightening of the membrane significantly enhanced the CO2/CH4 separation factor with increasing annealing temperature due to an improved polymer chain packing efficiency, which was confirmed by fluorescence spectroscopy and membrane density experiments. In contrast, cross-linking through decarboxylation occurred from 330 °C onward and induced a more open polymer structure for 6FDA-DABA-350 and 6FDA-DABA-400. Consequently, the more open polymer structure resulted in an increase in permeability of all gases. For 6FDA-DABA-350, a synergy was observed between the dilation effect of cross-linking and the tightening effect, causing a simultaneous, strong improvement of both separation factor (+100%) and permeability (+40%). As a result, the membrane performance crossed the 2018 mixed-gas upper bound and scored very close to the 2008 Robeson upper bound. Moreover, the cross-linked 6FDA-DABA-350 showed an increased resistance to CO2-induced plasticization thanks to covalent cross-linking.