I think (as is so often the case with preprocessed meat) it will often depend on who you buy it from. I often buy kosher prepared birds (which are always pre-brined salted) and I've never had any complaints. The nice thing about kosher preparation, is that it is done according to very specific rules, under strict supervision. It's not a factory process.
On the other hand, I cannot abide the brined birds from major poultry companies. Their primary goal is to increase the weight of the bird, not the flavor.
Factory prep seeks to minimize time and cost. They will use cheap ingredients (largely just water and salt, with no added herbs) and they will use whatever process takes the least time, most likely injection, which has the added benefit that they can force more liquid (and therefore weight) into the meat than would normally come from absorption. They'll likely insta-freeze it, so wait time is probably not an issue, but freezing causes its own issues.
I would say, for best results, take your time. A nice leisurely soak, real salt, and no pressure on the meat. For best results (talking turkey here) look for "Natural" or "Minimally Processed": if it says that, you shouldn't have to worry about competing additives.
I once left a whole New York strip in the walk in cooler for two weeks and it was great. Chances are that the salt and acid in the marinade preserved the pork well enough it would still be safe to eat after six days. I would check for slime and foul odor. There are several other factors involved such as how it was handled before put in the refrigerator. For example, a piece of pork that was at room temp for one hour with a semi sterile knife trimming it might not last three days. I always go with whether it has an off odor or not. I find that meat which has aged a little in the fridge will have a more robust defined flavor. If you run your finger across it and feel any type of slime just get rid of it.
Best Answer
Frankly - if you're aiming for pulled pork, brining at all isn't really necessary. Brining is most beneficial for lean pieces of meat because it helps the meat retain moisture and prevents a lean cut from drying out during fast cooking.
When slow-cooking a pork shoulder, you're going to wind up with very (deliberately!) overcooked meat fibers with much of their moisture squeezed out. The apparent moisture comes mostly from rendered fat and collagen converting into gelatin, which aren't really affected by brining; they are functions of the long and slow cooking process.
If you like the flavor that the brine introduces, then you can sure keep it, but given the other processes involved, I would not expect any difference in brining method to produce a noticeable difference in your pulled pork.