I'm not smart, BUT...
To answer the question, it's not how long the air path is, it's how much contact there is between the air you're trying to heat and the surface of the exchanger. Factors that affect this are the surface area of the exchanger, and length of time the exchanger is in contact with the air (which is further influenced by velocity of air over the surface (draw speed), as well as the length of the heat exchanger).
Other factors to consider are the thermal conductivity of the heat exchanger, as well as fluid dynamics, of which I don't know enough about to speak with any confidence on.
I'm sure there that a formula can be written to describe the relationships, but again, that's a bit further than I'm able to speak confidently about.
I do plan on learning these things eventually, but if you're interested in them, why not study these subjects? They're all very interesting.
As for the supreme, the supreme's exchanger is made of aluminum, which conducts heat exceptionally well, and as a result, heating only one portion of it is unlikely to cause regions of uneven temperature.
Temperature is (unfortunately the wikipedia definition really needs some cleanup) an average of kinetic energy in molecules. I would read
this part, and any related articles until the general idea is understood.
Each variable should be considered independently, though heat energy affects temperature.
Essentially the temperature is not what causes volitization of molecules, it is enough energy(often in the form of heat) to cause the molecule to no longer feel physical/chemical attraction to the surrounding molecules, and the molecule gets promoted into the gaseous state.
So there's a large number of factors to consider to get a good understanding of the process.
That being said, if you want evidence about how unlikely you are to cause significant regions of different temperature on the supreme, you can ask an experienced welder to show you how to weld aluminum with a flame.