Federal law also has limitations for how much ABV can be added through the use of tinctures, residual alcohol in barrels and such, when the beer is for sale. I think it's .5% max or something. That does not apply to homebrew, of course, but that may be why it seems to be a consistent % increase from a barrel when we are talking about the beers we buy in the store.
I didn't know there were Federal laws regulating it, but I suppose it's good that someone regulates it.
The way I understood barrel-aging; it's done merely to pick up the flavors and characteristics of the barrel and whatever residue from the previous contents, and not necessarily to increase abv. As I said, BJCP states that an Imperial Stout is between 8% and 12%, but a lot of BA stouts are listed as 13.5% to 14.5%. Having never attempted to brew a BA stout, I wasn't sure how that higher abv was being achieved. If the beer is going into the barrel at 14% and coming out at 14.3 or 14.5%, then in my mind that is "barrel-aging." If it was going in at 11% and being boosted to 14.5% by simply adding bourbon, that seems more like blending to me, and not "aging." You can drop a shot of bourbon into a glass of beer for that result.
I appreciate the responses. Now I need to keep researching on how to get a stout to finish at 13% or 14% abv without being over attenuated and dry. From what I've gathered so far, the keys are; pitching a super healthy, substantial amount, of yeast, maximum aeration in the wort, adding yeast nutrient, and possibly yeast vitalizer.