The BEC will probably generate most of the heat. If you're going to run 7 cells on that ESC for a while, maybe you should consider an external BEC to help lower the ESC's overall temperature?
IIRC, my HV Maxx ESC's BEC is good up to around 9.6v (only one lead powers the BEC), so I would assume that's about the limit of the GTB's BEC, if not a tad lower.
I would LOVE to see someone run the GTB on 3s lipo safely (with a UBEC of course).