hot air dump?
I too have upgraded to the Robbin's muff a few years ago, and see a decent 50+ rpm drop now, over the barely perceptible stock tube.
a couple of things to consider ( I am not a thermodynamic engineer).
1. when the carb heat is 'closed', there really is no 'flow' of hot air to worry about....as ma pappy useta say...."if it ain't going out the back end, t'ain't nuthin' entering the top end."
2. the Robbin's design, while superior to the soup can, lets air in the slot, around the pipe, and immediately down the hose to the carb....giving it very little time to get hot. I am sure it would be superior to drill the end plates of the tube, and close the slot, especially in the centre. Lotsa guys have added stainless steel wool, or a coiled spring, to increase the heat transfer.