That's a bit of a two edged sword. Hotter temps are good for low HC readings, but, not so good for NOx readings if they get too high. That's why the EGR system was developed. Adding a bit of exhaust into the intake lowers the combustion chamber temp, and also helps for more complete combustion. At least, this is what I've been told and have found over the years. I'm just trying to figure out what made just the 25MPH NOx reading hit the roof. All the other readings are all well under the max allowed and haven't changed much from the last smog test readings.
