Before you jump one way or the other, Bob, would it make sense to discuss what you're shooting, the type of shooting environment, and your objectives with the longer lens on the 8x10?

For example, if you're doing landscape-type work, and the objective is to get tighter compositions in situations where you can't move the camera position, what are the other trade-offs? The added bellows of a triple-extension may introduce more wind-induced vibration or stability issues, offsetting the potential added sharpness of the 600C over a 600T. While tilts with a tele design are more fiddley than with a conventional lens, they are not impossible. But, with the tighter image circle on the tele design, you may not have room for much tilt, anyway.

Obviously, I'm adding more questions than answers here, but that might be the point. There are additional considerations beyond what has been discussed so far.