I am not an expert on the Behmor, though have used them for many years. I think the afterburner is just to reduce smoke and bad aroma's. I don't believe it has anything to do with the roast time or roast profile really. The heat in your beans comes from the quartz lamps NOT the afterburner.
If you are having issues there, might want to check cleanliness, and voltage coming in, and of course loose connections, and age of lamps will play into it too. Again, Id not point to the afterburners as a cause for increased roast times, unless you have a rather starved power outlet and the couple 100 of watts or so it pulls is dragging your voltage down. Without knowing a thing about your situational wiring etc, older homes with aluminum wire can have these issues if stuff gets loose over time.
Check voltages when the thing is running full bore. do a clean out run and see what volts are at.
Aaron