I have moved my cook temperature again, upwards this time to 300F, and this gives me a better overall texture and moisture. St. Louis Spares get done around 5 hours. I can get my cooker to 300F with my cast iron diffuser in, it doesn't seem to take away any heat. Are you allowing enough room between the diffuser and the sides of the drum to allow for good airflow? My cast iron pizza pan is about 3" in from the drum sides, it drafts well and the temperatures work great in this way.
Whip It Off, Chambers!
"perhaps...but then again...maybe not..."