I emailed Fostex in Japan regarding the driver with the MK2 - in Japanese of course, using Google Translate so not perfect. I asked if there was any difference in the driver of the MK2 and original TH900, since the specifications on the website were the same. Here is the answer I got just now from Fostex support.
Subject: TH900 MK2
Thank you for contact us.
My name is Fostex support team of SATO.
I would like answers for questions received TH900mk2.
The same thing in the TH900 and TH900mk2 drivers are no differences.
And changes in the TH900mk2 had become a removable system cable and
Purchase a balanced cable (sold separately) to the balanced connection is also available
I was like that.
Best regards.
So as per my suspicion since the specs (100 dB/mW, 1.8W max power on the driver & frequency response) were identical on the site and described the same exact way, there is no difference in the actual driver between the original and MKII.