Take what follows as good solid rumor rather than absolute fact.
1. Sierra jackets have always been a little more ductile than J4s. This from some testing done when we had some long-shanked 6.5mm flat-base bullets (on J4s) blow up. So, both Sierra and J4s were tested. I don't have the numbers, or even the name of the testing laboratory, so it's "rumor."
2. As far as quality control goes, I *believe* Sierra will accept a variance of .0003, whereas J4 is less. I've heard .0002. Having said that, I know of a good bullet maker who sent a lot of Sierra jackets back because the variance approached .001 -- over 3 times what was said to be the limit. Of course, that doesn't mean that all Sierra's are .001 -- or even .0003.
It kind of boils down to a matter of quality control, and as Mike Walker pointed out in one of his interviews in Precision Shooting, how hard you can lean on the suppliers of the sheeting. If that's not held tightly, nothing else will help. Remington, a big buyer, was able to require very tight tolerances, why their 6mm competition bullets were the "best" (and money-losers, so they stopped the production...).
All it will take from Sierra is a decision to tighten up a bit on QC. For all I know, they already made that decision.
3. In years past, Sierra didn't give their distributors, except maybe two companies, a financial break. That meant you either had to go through those two distributors, or pay a lot more from a dealer who had to pay more. I have no idea if this continues today.