Update:
Although the Smoke Daddy PID controller was maintaining +/- 6F, the fan "puff" operation (vs. continuous) may have resulted in the black soot I observed during long cooks (incomplete combustion?).
I switched to a Savannah Stroker v4.5 (with continuous fan operation) and the Savannah upgrade fan. After autotune, the temps are stable (+/- 12F) and no more black soot.
Truth be told, all of my Traeger upgrades have resulted in no perceived difference in food taste (the meat doesn't know the difference).
Internal top shelf
SS 9-hole burn pot
Ceramic 100,000 cycle rated igniter
SS 10g heat shield
SS 12g drip pan
SS 12g drip edge insert
Smoke Daddy auger
Smoke Daddy heavy duty auger motor (2RPM)
Smoke Daddy 35lbs pellet hopper
Savannah Stoker v4.5 PID controller
Savannah upgrade "quiet" induction fan
Door high-wind stops
Bottom leg shelf
Front fold-down shelf
Second temp gauge