Please do add the length of stack on. I really want to see if this has any affect at all. The reason I say this is the 2x was developed in the days of natural draft. Since you have a fan on the fire the natural draft is...well blown away. You now have forced draft fire; BUT I am still wondering if it will have a positive affect or if the added friction of the extra length of pipe will just increase the back pressure like in a trucks exhausted after a turbo charger is used. The longer the pipe the more back pressure.

Ben