GOOD INO.! I'm really curious what it may be about your one rifle that'd cause it to shoot "slow"... Barrel diameter way on the large side of the tolerance? I'm trying to think of what else may cause that. I'm quite sure you'd have noticed any problems with the chamber (what I can already tell I'm SURE you'd have noticed discolored brass!)
There are many "telling" signs when your getting even close to a high pressure load. A FEW of the things I look VERY close for are flattened primers (around the outside edge), or when pressures are getting high I've noticed the primer will actually show marks on it imprinted from the face of the bolt, coloration of the brass, unburned powder, excessive residue when cleaning the barrel (could be from many causes if your not familiar with that particular barrel / load), accuracy of the loads as they seem to "cross a threshold" (I've seen loads get more and more accurate as the powder load increases, then they all the sudden the accuracy falls back off). Stuff like primers that are pushed back around the firing pin, "waves" or warped brass, discoloration, and much more, are ALL signs that you'd better stop shooting RIGHT NOW! Those loads are NUTS! I've found cases at the range with pierced primers... No blood on the ground. Guess the guy was lucky that day!
I think you've had more than your fair share of bad experience with factory barrels. I FULLY understand your opinion! But I've seen some factory barrels shoot REALLY NICE! The biggest variance is from mass production companies (Rem, Ruger, Win., all of'em) over using their tooling. If you can find a barrel that was produced shortly after tooling change thats where you'll get your "sub zero" rifle from. Unless your toteing a borescope to the store, its mostly luck of the draw.
Give that factory barrel a fighting chance. Try cleaning up & chamfering the flash holes on both sides, put few "A" batch rounds through her with a cold bore and see what she'll do! "cold bore" may be the trick your looking for! If she starts thowing rounds as she heats up its cheap to get it cryogenically treated (my local shop will do it for $50). I've seen it make a skinny factory barrel on a Ruger outshoot a heavy bull barrel on the same gun! (yes, it was only a 10/22, but does that matter?)