It's been a while since I last looked at Wayne's plans, but if I remember correctly, the insulation he suggested should only be about 2" thick. That would explain the improvement when you halved your insulation.

BTW, there's no such thing as over bracing, so long as it didn't significatly change your cabinet volume or impede air flow. With an internal volume of over 5 cubic ft, you've got to install lots of bracing to make a significant dent in there.

Gar.