Restock, maybe I missed it but I don't see where you used anything but a jittery optical out compared to the USB.
I agree it looks like there is an excess amount of jitter in the toslink output of the Airport Express, Mac Mini and Transporter (at least compared to USB). But that is why I recommend to Henry not to got the Airport Express or Macbook to toslink route. By the way, I don't see why your source (Airport Express) has any less Jitter, just that you have a very good anti-jitter circuit in the Altman Jisco.
In any case, I would recommend (to Henry) to get the USB version of Benchmark due its upgraded circuitry and added USB output compared to the Standard DAC1, try the wireless setup with AE and compare it to the USB version. Best to make up your own mind about this and trust your own ears. The differences are subtle to say the least., but since the goal is to assemble the"best sounding Music Server regardless of price" it would be best to keep options open.
As far as the wireless 802 network vs. ethernet work debate goes, dropouts are going to be unlikely with an all Mac setup. you only might run in trouble down the road for playing back high rez 24/96 files. There I really don't see that much of a difference soundwise for 16/44.1 file playback.