Good news!
Figured out my airflow/heat problem. It was actually quite simple, and in retrospect, I kinda feel like an idiot for not picking up on what the problem was because it seems so obvious now. But Goo-Goo's post got me rethinking every aspect of how I built my fire and do my cook, and it turned out the issue was right in front of me.
So, as I mentioned, I use a chimney to start my coals rather than using a fire starter. I already had a chimney for starting briquettes and was well versed with how they work, so when Chino first mentioned it awhile back, seemed perfect. Goo-Goo talking about replacing the bottom cast iron ventilated grill plate with a basket got me thinking about ventilation in that area. So I quickly figured out that the problem was arising from a combination of the bottom grill plate and my chimney method. The issue is this: To start my coals in the chimney I usually grab some scrap cardboard and a handful of paper from our paper shredder to put in the bottom of the chimney, and light that up to get a good flame to catch the coals. The problem is that that would leave me with a big pile of fine ash from the paper and cardboard on the floor of the KJ firebox on top of the bottom grill plate, which would likely severely hinder airflow from the bottom. Basically, rather than having my burning coals directly on the ventilated bottom grill plate, there was a layer of fine ash in between preventing any ventilation. I picked up on that this morning when lighting up. So what I did was, before dumping the coals out of the chimney, I picked it up and scraped all that ash to the sides to clear the holes. The result was a ROARING fire. I wanted to let it heat up a bit to do a little cleaning before starting the cook, and it got up to 575 with the heat deflector plates in before I started choking it down. (probably actually hotter, but that is what the dome thermometer was reading. When I was cleaning, the heat was intense. I'm sure if I took the heat deflectors out, I could have gotten a nice sear on some tomahawks.
Anyhow, problem solved. Glad I figured it out and that it was so simple. I still can't believe I didn't catch on before now, but I guess that's the thing with grilling/smoking/bbq. There are lots of variables, and it can sometimes just take time to learn some of the seemingly most obvious things about your equipment.