....but the point being that the NG does not fall out of the sky at Vref-5 and presumably does not suffer from this legendary 'winglet stall'....
You won't necessarily "fall out of the sky" because a relatively small part of the wing has stalled.
This is NOT B737-specific, but in a general sense, it's entirely possible that at the higher AOAs associated with the final stage of the approach/flare, the winglet is sufficiently off-design that there is a flow breakdown, or stall; if the winglet has been "tuned" for maximum efficiency in cruise, then the conditions at the tip may be very different. I certainly have heard of similar behaviour on other types (winglet stall, that is, not the control issues mentioned).
Personally, I would be surprised if there was a significant handling effect, as
(1) winglet retrofits have generally been accomplished without having to introduce handling limitations, and often on the basis of no change to handling (not perf, otherwise why bother!)
(2) if there were an effect, there would be numerous issues relating to asymmetric versus symmetric stalls, and I can't again recall of limitations which might be consistent with such a concern (xwind limit changes, etc.)
Again, personally, I'd be more inclined to attribute changes in control characteristics in the final stages of the approach to increasing ground effects, but that's just an opinion.