IMO, a peak Borg with a modern frame would be the best player in the World right now. Federer is past his peak, and IMO, peak Borg was a better all around player and athlete than Nadal or Djokovic.
Borg would thrive with a modern frame. He hit heavy enough topspin with a 65 si wood frame and gut strings strung at 80lbs. Imagine Borg with a PDR strung with ALU at about 60 lbs. The prospect is fascinating to say the least.