I've played legitimate 13 greens in a tournament. It was playable, but it sure gets you worried about what happens 4-5 feet past the hole. You definitely get a number of borderline shots and some really crazy putts. Luckily the course was set up well.
To me once greens get above 11, you have to be very careful with hole locations because there is tremendous potential for goofiness. At 15 only the flattest spots would be pinnable. It would be playable in those circumstances, but unplayable if the setup were overzealous or mismanaged or if it was windy.
Legitimate 11 greens are not crazy to competitive golfers. I'd say that crazy starts at around 13, depending on how slopey the greens are.
I agree completely, Matt.
I own a stimpmeter and have played greens at 12 and 12.5 in SCGA tournaments. The SCGA Golf Course, in fact, has some of the fastest greens in California during the winter and spring months. 12.0 is easily achievable there. At Mission Viejo CC for the SCGA Mid-Am this year, I stimped the 10th green at 12 after my final round.
12 is certainly puttable and I've played some of my best golf on greens that are in the 11.5 to 12 range. 13 would be very, very, very fast, but also still puttable. If Faldo says the greens at Muirfield were at 13, then I believe he was probably close to the truth.