A short follow-up is in order, now that I've shot with the lens for almost 3-months.
Not only do I notice an improvement in my image quality compared to the 500/v1, but others are commenting that I "did something." My processing is unchanged, so it's all in the lens. I still believe, "It's the photographer, not the equipment", but when you eventually need to break through a ceiling, this piece of equipment will move your forward.
I can casually pop off hand held, 60% crops of the moon that stand up to really close inspection, like this:
Waxing Gibbous Moon © (Explored) by
dcstep, on Flickr