We are still talking about subjective impressions. Only number i saw is a suspect number on a wikipedia article: Nalbandian 110 miles per hour. Don't know about real measurement of ground strokes. But what i can say, is the follwing speculation: If Fat Nalby can make 110 miles, i cannot deny, that Rosewall made at least on occasion 80 miles. He faced some servers with the hardest serves in history, including Pancho, Ashe, Dibley or Tanner. And if he hit a perfect bh with the sweet spot in the middle if his racket head, even with wood or steel, his bh would lose only a few miles on the return. You can hit hard with wood, but the problem is consistency, because you need perfect timing with the much smaller sweet spot.