All 870's and other modern shotguns have forcing cones. The dufuss at Remington probably isn't quite sure from which end the shot load exits. He or she probably does know which end of a golf club to hang on to. Duh, welcome to Remington Country.
To prove the above statement, take a look through the chamber end of a new Police Magnum barrel. They have reamed the poor thing to oblivion. The forcing cone reaches half way up the barrel. (OK, I exaggerated a bit) Why would they do this if it doesn't enhance the performance?
Could it be because Vang Comp and other custom gunsmiths have been doing it for years?