Hi,
The packing density will affect temperature also. One LED standing three feet away from another LED in free air will not affect that second LED very much. But one LED mounted 0.5mm next to another LED will raise the temperature of that second LED by some amount.
Furthermore, six LEDs mounted around a single center LED will raise the temperature of that center LED more because it has air that could be heated now than when it stands alone.
You could try mounting an LED by itself, driving it with a constant current, and measure the rise or fall in voltage over time starting the instant it is turned on. You could then mount the same LED where it is supposed to go in the board, drive it with the same current, turn it on, measure the voltage again over time, then compare the final temperature (rise or fall) to the first test to see how much difference it makes with the different the mounting patterns.