High Current Pulse on LED
The demise of LEDs occurs typically not due to overcurrent, so much as overheating due to the overcurrent. 

Cree XLamp LEDs, as also some other high intensity LEDs (e.g. Piranha), are designed with special consideration for rapidly drawing heat away from the light-emitting semiconductor junction within the LED package. This allows for higher pulsed currents than designs with less effective heat extraction.

If the overcurrent pulse duration is short enough, and blanking duration for cooling off long enough, even the cheapest and most commonplace nameless LEDs can be driven with pretty extreme currents, well beyond specification. 

The point to note is that duty cycle alone isn't enough to determine safe current limits, as even 1% duty cycle could be catastrophic if individual on-pulses are long enough to fry the LED, even before the blank pulse comes along to cool off the junction.

To determine how far an LED can be driven, experimentation up to the point of letting out the Magic Smoke on at least a few LEDs of a batch, would be the way to go.

I have successfully pulsed nameless 3 mm red 20 mA rated LEDs with as much as 100 mA, using on-pulse duration of 100 microseconds at 1% duty cycle, but there isn't really any practical reason to drive LEDs that hard, is there?