[IamIan]

Sense the high temperatures are a product of the rate of energy flow being soo tightly confined to that small volume/area (heating and cooling)

How about spreading that same power (energy rate) over a larger area .. it would dilute that same exhaust heat energy over a larger area .. thus if the cooling were the same , it would yielding a lower temperature hot side for harvesting .. of course leaving it small for the cat which needs the higher temps .. but the rest of the plumbing doesn't need the higher temps .. The larger surface area , might also make mounting easier , as well as more of them on a given exhaust line .. and starting with a larger hot side , also means a larger cold side , thus also increasing the cold side energy flow rate in the same outside environmental conditions , and with that increase it would also further reduce the hot side temperatures.

The other side benefit is decreasing ICE back pressure from the larger plumbing.