I am sorry, but I gotta call B/S on this. The exhaust gas doesn't really 'flow' so much through this. My understanding of the purpose of having the exhaust to gas tank line is to provide pressure to help force fuel into the carb. I could see if there was a constant flow through the line, it may (and I stress MAY) help a tiny bit. Not enough to affect the temperature of the fuel any appreciable amount. If you really want to affect the temperature of your fuel, store it in an ice chest until you are ready to use it. Another option if you are worried about exhaust temp on your fuel is to shield your fuel tank from the pipe. Mcmaster-Carr has all kinds of heat shield type materials for next to nothing.
I could be way off base on my evaluation of that piece, but I would bet that physics would prove me right.