A couple of things, and a further question.

Wren (Mike Murphy?) published a couple of articles on the augmented thrust of slightly divergent pipes (larger end of the cone towards aft) in RCJI a number of years ago, along with tests of the proper distance from turbine tail cone to bell mouth opening. Their pipes for the smaller ('44 and '54 sized) turbines seem to be largely single walled .007 or .008 SS, for the cone (remains cool anyway) and the straight segment towards the rear (that gets hot). They use a 1.5-2 inch 'extractor' double-walled segment at the exit of the aircraft, with 5-8 mm corregated or waffled SS to separate the outer from the inner part, with outer part extending 5-10 mm aftwards of the inner part. Pipes for larger thrust turbines are not usually divergent, as they presumably don't need the augmented thrust from a venturi, maybe run hotter and need perhaps full length double walls.

Another good question is does anyone know about the design and construction of double exit pipes-eg F4, and the A6. What thrust losses can be expected compared to single straight back pipes, what angles can be used and how heavy must the SS be to tolerate the heat just aft of the turbine? I think only Tam makes them and might have the experience to comment on them.