Witcher 3 runs at native 1080p, Ultra Quality, at some 30-35fps on equivalent SW2 level GPU. So indeed, straight PS4 port would probably be most cost efficient route.
CDPR, being CDPR, probably won't do just that (though I doubt they would handle the port themselves), they will probably leverage resolution to add some feature from Next Gen update, which indeed can be very taxing with all the RT thrown in.







