I installed a gasket on the door on my Ironwood 650 when I put it together. I don't see a downside to it, unless the door gaps were purposely left to encourage air flow. To me that would make sense if air/smoke wasn't exiting from that area. They created the downdraft chimney on the IW and T series to encourage a convection air movement. Having air/smoke escaping from the door defeats that in my mind.
Maybe I am wrong.
I have no visible smoke exiting the door on mine, and that to me is a good thing
I installed it to help maintain temps since I’m in Ohio and we can have all summer, winter, spring, and fall all in the same week. I can’t imagine there are any cons unless they made the lid so there is so airflow.