How did you test for "done" after the foil? Seems like it didn't spend enough time there, in the foil. Not clear on how you figured it was time to pull.
For reference. I have done 4 "high heat briskets" in the last 6 months. In fact they were the only briskets I have done on the WSM. They were huge, thick, minimal trim "first cut" flats, not even full packers. About 7-8 lbs.
They all came out fine following the pointers and references on this site. Used different marinades, sauces, etc each time. They all came out great.
Note that my "high heat" was around 275-325 for the duration of the cooks as I wanted to slow things down a little until I got the technique down a bit.
Last two cooks were actually exectued at 325 deg with an ATC, I even tossed these back on the WSM at higher heat to firm it all up a bit. And it was still great. I got a lot of good feedback from all 4 cooks.
What I'm saying is that in the end as long as you were checking for done in the foil by using the probe test rather than temp or time you should have finished just fine.
I'm guessing you were under cooked. Don't pull it from the WSM until you can pass a probe, knife or other implement through it without resistance.
I'm guessing you were under cooked on this one - don't give up.
