The technique of heat recovery from exhaust gases has a long history. In the days of open hearth steel making they created alternating exhaust paths called "Checkers". The checkers were stacks of pyro brick that heated up on the exhaust cycle and preheated the air on the intake cycle.
Counter current heat exchange is used and any number of industrial processes to improve heating cycles. It is also used in the feet of migrating water fowl to keep from freezing in ice filled ponds.
In my own experience I have used Black Iron pipe ( not galvanized pipe) with propane in a blower system to increase the temperature I achieve. Black iron is relatively inexpensive compared to stainless steel, and because it is thread-able by most suppliers it can be assembled in manifolds without welding. This makes replacement of parts a days work not a weeks. I used straight pipes not coiled, and of a large diameter to slow the flow down with out reducing volume. I didn't worry about the iron pipes because it is kind of like the "Mr. Wizard" trick of boiling water in a paper cup over an open flame. As long as air flowed through the pipe was ok despite the temperature would have melted steel.
For a wood fired system I have been toying with designs using two different sizes of fire place flue tiles. One outer and one inner.