You're quite right, you'll be getting more airflow on the open road. However, there still has to be a limit to it, although that's going to be depending on a lot of factors. ambient temp, power, how much power is being used, how often, and for how long. I honestly don't know the answer and short of sticking a temp sensor into the outlet of the cooler i'm not sure i will know.
However, that being said, Sean did recommend a new cooler. And i'd like to get one now that will be good for 400kw down the road. So I dunno.