I think that Jim is right. I use handlaid #8's for my mains, but I am also using big steam and long heavyweight passenger cars. They would move well through #6 turnouts, but not at limited speeds. I use #6 Peco Streamline Code 83 in my yard with no worries.
A "safe" speed is dependent on the items using the turnout. Box cars, hoppers, tank cars, shorty passenger cars....you can do track speed all the time with #6 turnouts.