This is an old thread but I thought I would add my two cents worth anyway. A flue heat exchanger will work a heck of a lot better with a turbulator installed. If anyone has ever looked in the flue of a gas fired storage hot water service you will have seen one of these. Other than in hot water services, they are mainly used in industrial applications where heat transfer has to occur, but the medium is moving with laminar flow. A turbulator breaks the laminar flow and mixes the medium so that maximum heat transfer can occur. The hot water ones are usually pressed out of aluminium sheet and twisted into shape, but a serviceable one can be made from fencing wire, however it will not last very long. Stainless steel wire is better. Google the term and see what I am referring to.
The only drawback is that installing one will slightly reduce the ability of the fire to draw due to both impeding the hot air flow and reducing its temperature. This is simply rectified by having a slightly longer flue.
In connection with this thread, it has been mentioned that having a wetback or other form of heat exchanger can cause some smoking to occur. This is also easily rectified by installing a stove pipe catalytic converter at the base of the heat exchanger. These things are available over the internet from North America at a good price.