Tubing used for RMH?

I am new here and am looking into RMH's.
My background is in the automotive business. Specifically exhaust. I.e. Headers, catalytic converters, mufflers and tailpipes. My question is has anyone tried to use automotive exhaust tubing in their designs?
There are many a thing that muffler men know about exhaust flow and heat exchange ( although using an expensive catalyst such as platinum and rhodium to change the chemical structure and raise the temperature of exhaust is inefficient No matter what the EPA and CARB have to say).
As the exhaust travels down the tube it comes in contact with every ripple, t joint, bend and dent which slows convection. I understand when making the duct for the mass you want as few turns as possible. The straighter the better. But if you were to use automotive tubing that is mandrel bend( smooth bent tubing that does not change size or shape in a continuous radius) elbows and u-bends(180 degree elbows) you could have a much smoother transition in the corners which in turn gives you more linear feet of exhaust tubing without loosing convection. Giving you or time to absorb heat out of the tailings before it leaves the system.
Thanks for your time.
Any thoughts?

Mike ; auto exhaust pipe is not readily found in 6" or 8", also cost would be a huge factor if you could even find pipe that size. A 5'long piece of 8" hvac pipe is $11.00.

You are right about the 8" stuff. But as an outside sales rep of all things exhaust and truck accessories I sell 6" systems for big rigs all the time. You are also right about cost. 6" tubing that I sell comes in 10' sections and to my customer at jobber price is $106. Almost $11 a foot. But if money was not an issue would it actually work better? If there was a build-off up at Weaton's place would mandrel bent tubing prevail?

In the case of a 6" system, certainly smooth bends would reduce but not eliminate the "elbow penalty". This might result in a longer theoretically allowable duct length giving more heat extraction, but I doubt whether the added cost would be repaid. If friction losses are critical, there are methods like the half-barrel bell that will help at minimal cost.