David,
You should simply try some bullets that you'd like to shoot and see for yourself. I have a 10 twist and shoot the 105 A-Max at 3060. The calculators say it has "marginal stability" at the altitudes it will be shot and shows also a slight decrease in BC through the 10 twist.
First hand experience shooting it tells me that it works fine. Well, actually better than "fine"...
710 yards
760 yards
You'll never know unless you give it a go...