OK - if you mount the temperature probe at the outlet of the MLT, that means that it will be warmer than the set point temperature between the probe and RIMS tube. Once the element heats the liquid, it's cooling off until is goes by the element again and is heat is added again.
I know I'm probably only talking a couple of tenths of a degree tops, but stratification is also there and more of a problem unless you constantly stir. My PID thermocouple is in the top 1/3 of the grain bed and I have a mechanical thermometer on the outlet of the MLT. At a steady state, there is a couple of degrees difference between the top and the bottom, the top is hotter. When doing a temperature step, the top of the grain bed will heat faster than the bottom, but the bottom will finally catch up. These are all observations on my RIMS system.
If I had the temperature probe in the outlet of the MLT and mashed at 154f or 156f, I'd be a bit worried about denaturing enzymes because the liquid going through the RIMS tube could approach 170f. I'd rather have it at the outlet of the RIMS tube or in the mash itself.