From what I've gathered following this post, I'm going to stick my neck out here. I'm going to say the boiler is poorly designed for coal. I'm thinking if the 3" water tubes were smaller, and more of them, you'd have better heat transfer, due to more surface area. Here's my theory; The gasses are comming off the fire, and going up, and out the flue. On this path, the hotter wood gasses are rubbing the surface area of the two 3" water tubes you say cross the boiler/firebox. Heat is being absorbed, and boiler temp rises. The coal gasses take the same path. Because the coal gasses are cooler, not much heat to transfer. Which all goes back to what we've been saying all along. The coal fumes needs to be held/trapped, and slowed way down in the firebox. They are simply passing through, taking the BTU's along for the ride. If you were to put a baffle up high, inbetween the two 3" tubes, and the smoke outlet, I think you'd see a big difference. Lowering the smoke outlet, or extending the one you have down, will also trap heat. Your draft needs to pull the coal gasses down, and then out the exaust pipe, leaving the heat trapped in the boiler. Apparently, the closed 8" damper with 2 holes in it is still not slowing things down enough. Remedy: You need to trap the heat in the boiler, and install a barometric damper.