I've owned this SC and this thing has never, ever, lost stability in tuning. In fact I re-tune it
less often than my current ME Quatro which equipped with Phase III lockers, to be honest...
I can't compare the tonal difference between vintage and Phase III as they are on different guitars, let alone the weight, hahaha. I can only say both guitars sound good and also resonate well.
I have no idea what's the real reason behind this change, but pretty sure it's not about weight nor tuning stability as I've proved both by myself. I tend to believe it's all about tone. This brave change must have some solid reasons supporting it. Maybe it has something to do with new TCI-tuned electronics, who knows... Please say something uncle Paul!!
And for me, if I had to choose a 594 I'll be fine with any type of tuners said above.