Can I choose a heatsink based on its thermal design power (TDP) for cooling LED chips?
Linear de-rating would be appropriate since the main cooling mechanism is Newtonian cooling, not radiation.
So let's look at a heatsink with 75W TDP, between an 80°C device and 30°C ambient, and you want to keep the LEDs down to 60°C in the same 30°C ambient temperature...
Now the heatsink sees a 50°C temperature rise at 75W TDP, (or a thermal resistance of 0.67°C/watt. Thermal resistance is the usual way of rating heatsinks other than CPU coolers).
If you decide to allow a temperature rise of 30°C for your device (60°C LED - 30°C ambient) ... that is 3/5 or 60% of the heatsink's desgn point : it follows that it can provide 3/5 of the 75W cooling, i.e. 45W.
(Crosscheck using the thermal resistance : 0.67°C/W * 45W = 30°C.)