Sure wish I had found your thread earlier. However, I have made very similar modifications to a New Braunfels offset smoker that is similar to the Brinkman, but made before Brinkman bought them. My unit is much smaller with a 12" x 24" grill, but cooks plenty of food for my wife, me, two middle aged kids, and four grand kids. It does require a supplemental rack that site over the grill though. Photos are the original unmodified, the smoke baffles, the tuning plates, and the final insulated smoker (missing the stack).
Differences between yours and mine are mainly:
1. I insulated mine with 2.5 inches of house insulation. See the picture. I painted all surfaces under the insulation with high temperature grill paint, covered the insulation with aluminum, but left the ends open for ventilation and to avoid moisture accumulation. I cover it when not using it.
2. I bolted two 1/2" angle iron sections front and back about 1.5 inches under the cooking grill to hold the baffles "tuning plates". This leaves me room to put a water pan on top of the charcoal grates, under the baffles, and at the entrance from the firebox. This works very well.
3. I put on a 36" stack extension to overcome the extra resistance of the tuning plates and to give better temperature management.
4. I drilled four 1/8" holes in the lip of the lid and three in the top of the cooking chamber to monitor the temperature with temperature probes.
Results are that I use about 1/3 of the charcoal I used to, reduced temperature variations top to bottom and side to side inside the cooking chamber to about 30 F (down from 130 F).
Anyway, yours is better but mine works real well for us. I am a real fan of the insulation It minimizes the effects of temperature changes, rain, etc. and minimizes ash buildup. It can maintain at least 325 F for 4 to 4 hours with a single fill of charcoal and very little attention using the Minion method.
I would like io hear experiences of others on these offset smokers. I really like the flexibility of adding smoke, adding a bunch of lighted charcoal to get a post sear on foods, etc.