I have trouble believing that the converter would cause your car to overheat. But, on the other hand, it is possible. Converters designs and placement are precise. They only operate after reaching a certain temperature, and therefore, are located close to the engine for quick "light off" but not too close so they don't get too hot. So, my point is that they obsorb a good deal of heat. Now with just the resonator, the heat has nowhere to go except to your block and heads, and eventually, your coolant. When I had a flowmaster and bigger pipe put on my car, flowmaster had a warning about placing it under a heat shield. It mentioned that the higher flow would make the exhaust components hotter, and the engine cooler. In your case however, flow hasn't increased greatly, and nothing more has changed except a piece that obsorbed heat, is now missing.
Tony