Trying out the amnps mailbox mod for the first time on my masterbuilt two door. Got 4 whole chickens going. Pit holding @ 240*.
I had a probe in one of his birds and another in mine. both were in the breast. Im thinking his came to 165 sooner because his were on a lower rack and they cooked a little faster. My masterbuilt is a dual fuel, and I run it on propane and can get her to around 325.Make sure you cook to temp not look or feel, especially if you're smoking for neighbors. The proper IT for chicken is 165 in the thigh or breast. At 165 the thighs will still be pink, especially if smoked.
Try spatchcocking your bird next time. It lays the bird out so you have more surface area for smoke to adhere to. It also spreads the bird out so it will cook evenly.
Since your MES can't get up over 275, make sure that the skin on the bird is super dry. This will help it crisp and it won't be rubbery. Air drying in the fridge uncovered for 8-12 hours is best. In a pinch a hair dryer on low right before going in can do the trick too.