BluBubble Boy, have you considered the other factors that cause soft results? If not, have a look at the shutter speeds used to capture those shots you claim are soft. Even with SR on, ensure that Tv does not exceed 1/focal length,
i.e. keep shutter speed faster than 1/300 for a shot at 300mm.
Also ensure that SR engages before you release the shutter - this involves keeping the shutter button half-depressed after locking focus momentarily until the SR symbol lights up in the viewfinder. When shooting on a tripod, use both SR off and the 2-sec shutter release delay function to avoid inadvertent camera shake from mirror slap.
If your tripod isn't rock-solid, that is a more obvious cause of camera shake and thus soft results.
Focus points are important as well, and any slight movement of the camera towards or away from the subject at high focal lengths can dramatically affect the sharpness of the subject in the image.
Overall, the 55-300 is an excellent lens (for a slow telezoom) and can give you great results. Some of my examples at full-stretch: