My chain is ready to be replaced according to gauge, but, my shifting is flawless. Should I replace the chain or squeeze another few hundred miles out ? It currently has about 2500 and is a CN6701. Thanks !
The current smoothness of the shifting isn't usually the deciding factor regarding when to change, but rather the cost of the other components that wear faster when the chain is worn: the cassette, and the front chainwheels. The idea is to replace the chain (relatively cheap in comparison) before it starts to damage the more expensive cassette and front chainwheels.
In my experience I get about 2,000-2,500 miles out of a CN-6701, and I'm riding in near-ideal conditions (Southern California weather). I'm pretty careful about keeping my chain clean and properly lubed. I typically replace the cassette about every three chains, and the front chainwheels (Ultegra as well) are still OK.