Yeah I believe John replied that the output impedance is a result of extra ferrites (IIRC) that were added just before the output to help minimize/prevent electrical interference (cell phones, etc). I've yet to run into any issue, though my cans are 70ohm so not as far off the rule you mentioned. John also mentioned that you could request him not to add the ferrites on a custom build if you were concerned about the output impedance.
Alright, thanks for the impressions. Since nobody seems to have commented on how it impacted their sound, I'm guessing it's not a huge deal.