Terra cotta has a fairly wide firing range. The addition of grog extends that by a couple of cones. It tends to get more brittle as you fire it hotter, and based on the absorption numbers they show, I would say the top temp that is truly functional would be more like cone 3 or 4, not 6. Typically terra cotta is fired to low fire temps, like cone 04-ish, but I do know people who work with it at cone 3. It gets darker as you fire hotter, and you won't find commercial glazes for cone 3. You'll have to decide what's going to work best for you among all those factors.
