App note from CREE on driving LEDs over its specified current capability. Link here (PDF)
The Applications Engineering team at Cree is often asked whether it is safe to operate Cree XLamp® LEDs with pulsed currents above the maximum data-sheet rating. This question is usually asked in the context of legitimate product requirements such as those posed by emergency-vehicle applications, specialized stroboscopic illumination and even pulsed modulation for general-illumination dimming applications.
The short answer is “it depends.” Multiple variables affect both initial and long-term performance and reliability of an LED. These include thermal resistance, pulse duration, as well as current amplitude, frequency and duty cycle.