Rods that are graded F (Bullseye Compatible for kilnforming) are expected to perform to similar standards as our F grade sheet glass. The FAQ page of our website has information on "what to expect from Bullseye sheet glass" and "What to expect from Bullseye rods", and you may want to read both these articles before proceeding. http://www.bullseyeglass.com/products/faq/#faq_018
Keeping your glass at 850 (degrees C) for 8 hours exceeds the testing that we do on the glass, so we would recommend that you perform a test before firing your whole piece. The glass may very well perform wonderfully when held at process temperature for an extended period, but it also might change.
What color styles are you planning to use?