The 600 watt rated power provided at the Titan Elite website is obsolete; it used to be the rated maximum in an early revision of the firmware until we’ve resolved certain stability issues at high power levels in a newer revision. I am going to ask them to fix that.
Regarding the power capability: the relationship between the DC current and the phase current circulating through the motor is rather complex; you can get a brief overview of the problem in my short post “Power balance explained”. To size the ESC properly, you need to rely on the required power output (dictated by the motor and the propeller), the required voltage at the rated speed (dictated by the motor KV parameter (RPM per volt)), and the required burst power capability. Having that data and relying on the relationships explained in the referenced post, you can come up with an estimate. However, if your propulsion system relies on low-cost poorly-characterized motors, you are probably better off just collecting empirical data by testing your setup on the bench. We can also assist you with tuning Myxa for your specific propulsion hardware to optimize its energy-efficiency and robustness; please let us know if that is relevant.