The performances of a small scale subsonic chemical oxygen-iodine laser have been improved, the output laser power being increased from 5 to 34 W, with the same gas pumping system. At this power range, we have shown that the modeling of the laser must include the temperature effects on the reaction kinetics. Our model shows also that the addition of a buffer gas such as SF6 achieves a significant cooling of the active medium, which in turn allows a better laser power extraction.