I think my boys may have answered my question about how high they can fall from without getting hurt. The other day, Beowulf was standing on top of the refrigerator and he saw me coming and leaped off, landing on the kitchen floor. I've seen him do it twice now, and while it makes a ton of noise, he seems none the worse for wear.
Of course, I'm not sure how bad it would be if he was thrown off after wrestling with his brother, rather than jumping off of his own volition. Would he still land on his feet? Also, I have to wonder if jumping from that height has long-term effects on a cats' joints. If I had to jump and land on my feet from that height every day, I'd get sore knees and ankles. Are cat joints that different? I hope so.