This is truely one of those situations in which
search is your friend. There have been a number of threads touching on this lately, particularly regarding the Beyerdynamic DT880 and DT990. The consensus of a handful of folks who have compared them appears to be that if you have adequate amplification, the 600 ohm beasts are somewhat of an improvement over the standard 250 ohm version. In the case of Beyers in the U.S. this means a considerable cost premium to get
the custom-made version.
I can't offer any first hand observations in response to your question.