I say down low, if the air can move out without going OVER the painting area I think you are better off. Again, this is NOT the proper way a booth works. With a real paint booth the air moves right past all your work. With a down draft like we have at work almost the entire roof is the intake and a giant filter. The exhaust is UNDER the car. It has a big hole under the car with a piece of metal covering the center of that hole leaving a gap around the edge of an inch or so, all the air comes down from the top and around that car with the overspray being taken away RIGHT NOW away from the work and going under the car.
But we are talking multiple filters that it is going thru. We are also talking a pressurized system where the air is is being pushed into the booth and being sucked out in a happy little dance. So it is quite different.
At home, in my personal opinion unless you have a LOT of air movement and a really good filter system you are best off to just get the damn overspray out before it falls on a wet finish and flattens it out.
You have gone to such lengths I am thinking you are pretty close to having a pretty decent "booth" and are doing a heck of a job. Where you go from here, I don't know. That dirt in the paint was coming from somewhere! Maybe do some kind of testing with smoke to see where the air is coming in? Maybe it is skipping the filter somehow. It looks like you have done a great job at making it air tight but maybe there is something you missed? But more likely it is coming thru the filters. I know old school was wetting the filters with water. I remember an old cross flow booth at a shop I worked at years ago we had a sprinkler out in front of the intake filters that would be turned on before spraying.
This isn't so good anymore with modern high solids iso kicking paints though.