Benjamin's take on clocking can be found in here - scroll down to Myth #1-Clocking.
Regarding I2S, it was primarily designed for internal use - short runs inside of equipment, which may be a reason there is still no universal standard. When I owned Metrum equipment, I heard no improvement when using the I2S input to my Pavane or Adagio DACs. In fact, with my system at the time, I liked the sound of the USB input a little better, but that required taking the Ambre streamer out of the mix so maybe it was apples and oranges. With the Singxer SU-6 DDC, I do have the option of using I2S and adjusting as needed to match specific manufacturer's versions of I2S. Unfortunately, I have not had an I2S capable DAC here since I acquired the Singxer. I wish I had it at the same time I had the Tambaqui here because I heard from a dealer that the Tambaqui sounds best through the I2S input, at least in that dealer's opinion.