" guess the exhaust as will be cooler in part because they expect you you to use something regenerative like a turbo to extract more thermal efficiency."

What???????????????????????????????????????????????

A turbo does take "wasted" heat from the exhaust to create power...however the extra power creates extra heat plus it requires more fuel. If you are suggesting (I really don't understand what your trying to say) that a turbo makes an engine more thermally efficient...well I don't know..

I spose a fuel efficient engine may have a high thermal efficiency value..a high horsepower engine however...may not necessarily? be highly thermally efficient????

I know my first statement is pretty much on the money, the rest...well car engines are quite complex.