This Johnson Temp control will not work for the MES. It's temp range only goes up to 100c or 212F. The MES controller goes to 275F. Also there is a built in short cycle protection, this delays whatever is being switched from coming on for whatever the delay is set at. It is adjustable but doesn't say how short the delay can be set. A delay wouldn't be a good idea for the MES, you want the heat elements to switch on as soon as there is about a 5 to 10 degree temp fall.
As for temperature problems and the MES, I have had my MES since mid June and have used it about 16+ times, I have only had temp issues once. The problem was to long of an extension cord. On another cook with a big load of meat, it took more than twice as long to get to set temp, but that was because I didn't boil the water 1st for the water pan.
There have been a few complaints about temperature fluxation. 5-10 deg temperature difference are not a big deal, provided that on average the temperature is +or- 3-5 deg from set temperature.