I've got to say you are "playing with fire" by running sap feed lines through the flue gases directly. The high temps there, around 1000°, could easily boil the sap in the copper tube very quickly, particularly if you shut off the flow out. You run a risk of having boiling sap shooting out somewhere. Tubes of liquid in hot flue gasses is how steam generators are built, but they are ready to handle steam. Be careful!
I figured out the maximum possible efficiency gain for a preheater once. Assuming the sap was near freezing and it was raised to near 212°, I think the advantage was about 20% over cold sap coming in.
2x6 drop flue with AOF/ AUF
160 taps with sacks
Eden Prairie, Minnesota