I have done 2 brisket flats exactly as Bryan described....just the flat purchased at BJ's, both were in the 5-6 pound range. Each had a good 1/4" fat cap (maybe more) which I DID NOT trim. I trimmed the sides but not the cap on top. I dry rubbed it. I placed it fat cap down on the top rack and I never foiled or turned it or mopped it. Just let it cook to internal of 185 I think it was.
The last time I put it in a smaller dry cooler fat cap UP after double wrapping in foil, piled all the newspaper I could find on top of it, then filled the rest of the cooler with crappy bath towels and I left the Maverick probe in the meat. It was still at internal temp of 150 three hours later when I removed it.
When I sliced it up, I did have to trim some remaining fat which did not totally melt away, but that was no big deal. With sides, those 5-6 pounders would probably feed 8-10 people at my house, we had 6 people last time and still had almost 1/2 left over.
I've never done the full packer, but I might try one and use the point in some chili.