Has anyone thought of stuffing the "pipe in a box" heater boxes with stainless steel steel wool? Or expanded stainless steel mesh? Back in the 70s I used to buy this stuff for use in the nuclear industry. It was available from several speciality steel supply houses. Not all that expensive. The idea is to stuff your hollow heater box full of something that will slow the air down long enough to heat it up and at the same time act as a heat sink without corroding too fast. It won't heat up as fast as fins, but eventually it should get up to operating temperature. Just a thought, who's first?
I agree with the thought that the outer box should probably be stainless if you are going to wrap it with Thermotec or some equivalent heat wrap. I destroyed a perfectly good set of steel pick-up truck headers in search of optimum performance. The metal just fell off in large flakes in a very short perid of time.