I've had a door pop open on a Baron, and it does get loud. All of the charts
on the front seat and my jacket nearly got sucked out. The plane definitely
did not fly as well with the door open. The extra drag was very noticeable,
even though it trailed open only around two inches. I was able to get it
partially closed, and the acceleration was very apparent, but I finally gave
up and landed.

While distraction is an issue, the handling was a lot different.