Just to get back on this thread's subject line.... I feel like the new retina iMac has opened a path for a cheaper cylindrical mac pro?
Just the opposite. The retina iMac took the old $2,499 price point that the entry Mac Pro sat in for 3-4 years. Apple filled the "hole" they created when bumped the minimal entry point to the Mac Pro up to $2,999
As the retina iMac goes mainstream over time ( retina 21" and then dropping of non retina models all together .... except perhaps lowest, gutted edu focused versions ) then maybe there will be a price point for the Mac Pro to drop down into.
Not likely over next couple of iterations. For several factors.
1) Single storage drive ( hence pressure to crank the SSD capacity up)
[Apple could put in a m.2 SATA socket on Compute GPU and use the chipset SATA lines but I doubt it. ]
2) "Fire Pro". .... as long as using that to mark up the GPUs , I doubt going to sink low enough to do two and still hit old lower entry price points.
3) For next couple of years DDR4 isn't at the tail end of the maturity curve.
4) Intel prices aren't coming down in the Xeon E5 space.
Since a mac pro doesn't have a screen, Apple could potentially build the same innards that is in the retina Imac and put it in a cylindrical shape and make it cheaper.
That makes no sense at all. You are simply just trying to reuse the case "just because". There is no need for a huge three sided heat sink with iMac innards. Folks would likely have their underwear in even bigger twist if coupled the iMac GPU to a container that had a much bigger thermal envelope.
The Retina iMac uses the following $213 part
http://ark.intel.com/products/80810
The nMP uses the following $294 part
http://ark.intel.com/products/75779/Intel-Xeon-Processor-E5-1620-v2-10M-Cache-3_70-GHz
An $81 swing in CPU package cost isn't going to cause a huge drop in system costs or pricing.
It is the same farce "Core i7" is radically cheaper claim that has gone no where for years .... still going no where in making huge changes to Apple system pricing.