We report on comparative studies of different mixing schemes in a supersonic COIL which, when combined with improved operation of a jet singlet oxygen generator, led to achieving very high chemical efficiency. Spatial distributions of the gain, temperature and I2 across the flow were studied for transonic and supersonic schemes of the iodine injection in a slit nozzle supersonic COIL as a function of the iodine and secondary nitrogen flow rate, jet penetration parameters and gas pumping rate. The mixing efficiency for supersonic injection of iodine (~ 0.85) is found to be much larger than for transonic injection (~ 0.5), the maximum values of the gain being ~ 0.65%/cm for both injection schemes. Measurements of the gain distribution as a function of the iodine molar flow rate nI2 were carried out. The temperature is distributed homogeneously across the flow, increasing only in the narrow boundary layers near the walls. Comparison between different mixing schemes in the supersonic COIL shows that maximum values of the output power and chemical efficiency for the supersonic mixing scheme are about 20% higher than for transonic mixing. The maximum power for the supersonic mixing scheme is obtained for operation conditions of the ejector COIL when the secondary nitrogen flow rate is about two times larger than the primary gas flow rate. As a result of optimization of the O2(1Δ) generator parameters, maximum power exceeding 0.5 kW was obtained. The maximum chemical efficiency, ~ 33%, is the highest reported efficiency of any supersonic COIL.