Great post! I too have found that higher heat briskets are not quite as tender as lower heat cooks, and is why I went back to lower heat cooks after spending a couple years on the higher heat methods. It can still be tender at high heat, but not AS tender.
I think a great example was given on last night's Top Chef when the top 4 were competing. Kevin did a fast brisket and could not get it quite as tender as the judges had wished. The judges felt it was a bit more stringy than they would like. Kevin even stated it was done to his satisfaction. Had it cooked longer (meaning lower temps to keep from burning), it would have given up more collagen and been tender like the judges had expected.