I had similar problem, original MES controller would never get above 240-250'. I added the Aber PID controller, and using the MES as an insulated box. The Auber PID wired directly to the heating element and overrides the MES controller. I bypassed the high temp overload switch which is not recommended, but was easier in my case because I am not that technically adept. LOL. Since I am never away from home when using the smoker, I figure it's OK. There are very detailed instructions how to do it while keeping the high temp overload sensor in the system from Dr K and Tallbm I think.
Once I installed the Auber PID, temps went to 275 when needed, maintained temps needed when needed, etc. Took my cooking to a whole new level. I am forever grateful to the folks on this forum as always.