The temp difference is about right for an operating cat, but the rear o2 sensor still may not be "happy". With an Oscilloscope you shoud see the switch rates of the post cat sensor about 50% of the precat sensor assuming the sensors and cat are good, of course a very lean or rich mixture can skew these readings. Hard to measure the actual cat temp because of the shield/casing. A graphing scanner might give you a better look at the comparative o2 activity. It is not uncommon for coolant to damage a cat. An aftermarket cat is about 185 installed.