Box-stock, mine would shoot about 812fps. Not all 14.3gr. pellets are equal, some shoot faster than others (has more to do with pellet fit than just the pellet's weight).
Put a "power adjuster" and spring guide and the lowest adjustment gives 820, the higest 895fps. Usually i set it for 830fps and just leave it there (tested, that's the spot that gives the best combination of power/accuracy/shot count).