Correct me if I'm wrong but most IB IMC will roll most SB-e IMCs given the same DIMMs.
It depends on the SB-E chip, but in general, yeah, this is true. You know what is also true? How memory speeds barely impacts performance in CPU benchmarks let alone in the real world. People who need more memory will get more memory. You're also going to put more strain with 4-dimms on a IVB chip than on a SB-E chip, so that extra stress might make IVB memory clocks look a lot more like SB-E's, which still has 4 channels instead of just 2.
The problem with the logic here is that quad-channel doesn't help single-threaded tasks and faster memory speeds barely do anything to make the CPU itself go faster (over DDR3-1600,) so all in all you will only see the benefit when you start using multiple cores, which is where quad-channel memory shines.
Either way, I don't think the memory configuration will make a huge difference in performance but the places you do notice it is when you start stressing out all of the CPU cores at once on a memory intensive task.
One of the things I like the most about SB-E is being able to control the BCLK to hit just about any memory frequency I want. I'm not just stuck with the memory dividers at the stock 100mhz bclk so it gives me some granularity when it comes to what settings I have access to to overclock.
Is SB-E better. Not for the average user, no. For people who want it, it sure is and for all those people who invested in SB-E, we still have IVB-E lined up and I'm willing to bet that IVB-E will be plenty fast in comparison to IVB. Hell, even SB-E is pretty fast in comparison to IVB, but you also have to keep in mind that IVB came out after SB, it's on a different process, and quite frankly it's on a different platform too. Remember that SB-E has 856 more contacts than skt1155, not because it is cool, but because it needs more power, more grounds, and it has more PCI-E lanes and more memory channels.
Is it all necessary? No, not all of it.
Is it pretty awesome and fun to play with? I certainly think so.
I guess what I'm trying to say is, if you don't like SB-E. Give IVB a try, just don't expect light and day differences and think long and hard before you make the change because if you sell off your X79 rig, there will be no going back unless you want to waste more money than you already have at this point.
I don't think there is any reason to ditch X79. It works well and it's a worthy platform. Also for gamers, I think we will see games using more and more threaded resources and CPUs like an Intel 6 core might actually help.
Just out of curiosity, try turning off HT on your 3930k and see where it gets you. I know that my 3820 is a little faster on 4 cores when I disable HT as opposed to 4 cores with HT enabled.
I would be comfortable saying 95% of people even here or other enthusiast based sites, wont need more than 32GB and are likely fine at 16GB or even 8GB for as long as that platform would be useful (couple more years before most may want to upgrade).
When I boot up my VMs and start developing I've seen my peak usage hit 14Gb. I can think of things that would put me over that, but I try to not be too unreasonable with how much I have open at once.