How much heat is generated at highway speeds in 60 serries tyres, if its true that with nitrogen you could increase the PSI by say 2 PSi without having them over inflate on the highway, there might be some merit for short trips in fuel economy, by having a slightly higher inflation with nitrogen.
A complete waste of time, just stick with regular air.
Most of your tyre heat is generated by the brakes as well, not by the myth of the tyre sidewalls flexing and thus causing all the heat in the tyre.