I grew it for a couple of years and was able to overwinter it in a pot in a foyer that isn’t used in winter, colder than indoors and warmer than a NH winter. Other plants that have survived here include Salvia guaranitica Black and Blue and Dahlias. Other Salvia greggiii didn’t successfully over winter there.

I'm in Oklahoma zone 7b. Salvia 'Hot Lips' usually survives winter with some die-back of the top half of the plant which is fine because it needs to be trimmed back anyway. However, last winter they died all the way down to the roots. Maybe thats what it does in zone 6A but in my experience its not as cold hardy as Salvia greggii.

Maybe it depends on the amount of moisture as well as temperature?

It is an evergreen shrub in my climate (still in bloom as I type) so I'm not much help :-) But the websites I trust as having the most reliable hardiness data list it as a zone 7 plant. That should mean it is at least root hardy in that zone, possibly more. And microclimates in a slightly colder zone may increase its ability to winter over. As with any other salvia, drainage - especially in winter - will be key.

I wish Plant Delights would stop touting it as hardy to 6A. It's not.

I find that Plant Delights tends to err on the generous side for most hardiness issues. Since they are in a zone 7b/8a location, you have to wonder how much first hand knowledge they have of this plant in colder zones.