Why are you thinking about a flue damper. You've only got 16' of flue so there's no way it's overdrafting. The manual say 13' minimum so you are almost at the the minimum. If you are using a lot more wood than you think you should be you need to reduce the primary air.

m on Tuesday with outdoor temps in the teens, my 2000 sf was at 70 when I got home. I cleaned ashes (using the ash pan) and had plenty of coals to get regular size splits going in no time, no kindling required. My stove will cruise along for 4-5 hours on low damper settings at 450 then slowly come down to 300 and stay there for hours more as the big chunks in the back of the box burn off.