I have never done a high heat brisket. Gotta try that.
Briskets have been a hard thing to master. I'm still trying to get it right. Sometimes they come out great, and others... well, not so much. I normally buy prime packer or choice flats from Costco. Mostly I buy flats because it is just me eating it, and an entire brisket is just too big.
Below are some things I have found for cooking flats. Please keep in mind that these are just my opinions. Some people may disagree. Feel free to ignore anything or everything.
1. Most important: go by feel, not just internal temperature! Early on, I was so obsessed with temps and measurements that I was taking the briskets off too early. Use the meat temp around 185 or 190 as a starting point to check the feel. For a flat, the probe should feel like it's going through room temp butter. The meat should feel very "bendy" and supple.
2. Second most important: after it gets probe tender, let it cool down to about 170 before putting it in the cooler. The goal is to have the cooking process stop before resting.
3. Let it rest! After it gets to 170, wrap and put the brisket in a cooler for 1 to 2 hours. This makes a big difference for me.
4. Salt liberally the night before. Then add just pepper when you are ready to cook.
5. Last but not least: I have had the most success using really low and slow for flats. The slower, the better (within reason). I keep the WSM at about 210 to 225 for brisket flats.
Again, just some opinions from my trial and error.