Saw the string specs and tensions for players at the Australian Open the other day on the web. Just googled it. Seem to remember Serena Williams was really high. The lowest guys were Davydenko followed closely by Federer - 21/19.5 if I remember.
I played the other day with an old aluminium frame strung in co poly at 40. It was artificial grass, heavy balls shooting through low. The other team thought I was a ringer that had been brought in especially for the match. Pretty unique playing conditions but low tensions worked a treat