I'm lucky enough to have a few tins of recent JSB 8.4s which weigh from 8.28 to 8.55 and at 50 yards straight from the tin they are fantastic. I've had several sub 6mm C-C groups indoors at 50 yards, and not had any funnies at all in 2000 shots. Over the last 4 rounds of our winter league I've scored 115 ex 120, and one of those was hiting a twig - all straight from the tin.
I also have several thousand of various batches which are half as accurate after weighing.
I was particularly interested to see j.waterfield's group tests; I would certainly agree that slightly less accurate pellets in still air suffer increased drop and definately take greater and less predictable wind deflection. This results in far worse groups in the field and less targets hit.
If conditions double a 9mm indoor 50 yard group to 18mm that's unfortunate, when an apparently acceptable 18mm indoor 50 yard group doubles outdoors you have a group the size of the kill zone................