When I called ATI this week, they said with RPM's below 6k, there's no reason to change rubber durometer. Personally I would run a Fluidampr for RPM's below 6k, but that is not based on any 6cyl specific knowledge. I have read about tests not on a 6 that showed below 6k the Fluidampr is hands down the best. If you spend time above 6k, I would go with an ATI. 2 cents given. Panic's points are valid. Even a new stock one will help quite a bit.

"The first rule of overkill: You can never have too much overkill."
"Overkill is underrated."