Very cool. I wasn't aware of pixinsight before reading your link. I tend to take "recommended" configurations with a grain of salt. It may be that pixinsight runs better with a Xeon, but I would definitely check out their forums and benchmarks on their site to see if you really need it.
Yes, but so can consumer chips. there are 6/8/10 core i7s. Unless you're scaling beyond that it doesn't matter, especially since if I had to guess, this proposed Xeon iMac wouldn't venture into the higher core counts because of limitations already discussed here.
As for RAM, yes, the Xeons take much more RAM but that really doesn't translate well to desktops. Sure, the memory capacity of E5 v4 Xeons is 1.54 TB, but you're really only looking into these higher memory thresholds building servers or other big power hungry machines. Current consumer chips max out at 128 (at least according to intel documentation).
I'm aware of the benefits that come with Xeon, but in my experience, a lot of the time the proposed benefits are inconsequential to the needs of the user.
As for your dual chip iMac configuration, I highly doubt that will come to fruition.
