There is really no price reason to leave the E5-1620/50 for something like the E3-1245/80, the only reason Apple would do that is for TB and form factor.
There is a pretty big gap price wise. Two "Good" , "Better" , "Best" line ups core foundations ( v2 and v1 pricing but the v3 and v2 pricing coming up should be similar):
E3's ( Pulling iGPU xxx5 models to ease TB impelement. Is suspect v3 are bumped up on GHz. )
E3 1225 v2 3.2 GHz 4 cores $209
E3 1245 v2 3.4 GHz 4 cores $266
E3 1275 v2 3.5 Ghz 4 cores $339
http://www.cpu-world.com/news_2012/2012020701_Prices_of_Xeon_E3-1200_v2_CPUs.html
They probably need to do more differentiate on different PCI-e cards to separate these out more. (just like on the iMac line up only with desktop cards to push the overall system into higher range. )
E5 ( again three to )
E5 1620 v1 3.6 GHz 4 cores $294
E5 1650 v1 3.2 GHz 6 cores $583
E5 1660 v1 3.3 Ghz 6 cores $1080
http://www.cpu-world.com/news_2012/2012020701_Prices_of_Xeon_E3-1200_v2_CPUs.html
The 1620 almost starts off where the E3 1275 ends. Trying to limbo back down to the $2,000 border would definitely be possible making that as the foundation swap. Especially with the task of trying to bring the top end of the line up back under the $2,600 mark.
Xeon E3 has this. (somewhat similar to how the Core i 49xx has it switched off from the basic Xeon E5 architecture so to be a Core i offering).
over 300GB in max RAM capabilities
Pragmatically an evolutionary Mac Pro with one bank of RAM won't hit that. It isn't an OS X limitation and OS X's limitation is already busted by current Mac Pro let alone the E5 series (v1 or v2).
But a 32GB cap is going to snare a few. (but apple gets to put off upgrading OS X for another couple iterations )
and an additional 2 cores for the 1650.
Probably additional cores in two slots in the line up. I think more folks will not be happy with the GHz tradeoffs. The E5 1620 is what alot of "legacy software, single threaded" folks have been looking for and had to push to the top end of the single package Mac Pro line up to get when really only have entry level money ( hence the moaning and groaning.)
So for those that really do want to make some sort of mini cluster out of this product, pricing goes through the roof for the same capabilities already possible in dual processor workstations.
If want to do a mini cluster then Mac Minis are already there. If all doing is x86 core count chasing then just start racking minis. Especially if can swing quad minis through the whole line up on this next v3 (Haswell) update.
If it is a cluster of GPU core counts than doesn't work well, but x86 only really needs to be about $800/node not $2,000/node cost if trying to stick to a limited budget. If Apple can squeeze quads as the default into the mini then it just takes 5 to get to a count of 20 ---> $4,000. Which is much lower than any two 10 core count offering is going to be. That will be about
$2,200+ just for the two CPU packages themselves before Apple mark up and rest of larger infrastructure costs are added in.
But yes a very high speed interconnect is not there ( if need to ship intermediate results off the different nodes in the cluster). And the mega expensive per node software packages don't scale. Neither one of those I don't think Apple looses much sleep over missing out on.
I don't think Apple is trying to say "stack up boxes" to get back to dual Mac Pro at all. That's more of the ultimate modular speculations that any real insightful read on Apple's direction. Either they will ship a dual to participate or drop out of that sub-market. One of the two.