The dual CPU setup doesn't help Lightroom either -- it's not terribly efficient for multiple cores so I can't imagine it would do any better for dual CPUs. I'm trying to find the adobe faq on the topic I thought I've read about that.
Quadro has some merit if I wanted a 10-bit workflow since I already have a 10-bit panel.. but I did kinda want to dabble in some gaming and VR with this build as a side-benefit so I went with a GTX card.
This really depends on what you are editing. 13mp snaps from your ancient DSLR? Yeah, no point. 52mp snaps from your digital medium-format camera? You'll probably want more than you'd think. 4x5 or 8x10 negative scans? Open the bloodgates.
(and remember that by default Lightroom does not parallelize across multiple photos within a batch job, so multiply your measurements by 4x or whatever if you're going to be launching multiple batch jobs at once to fully occupy a high-end processor.)
128 GB is overkill, but there's a solid justification for at least 32 GB or 64 GB for power-user situations.
You would figure that's an obvious and simple feature to offer, at least as an option.
Overall, with your Silicon Mechanics build they'd probably be closer to a $8000 build. On top of that, a P4000 is pretty much a beefed up GTX1060, which gets smoked by a GTX1080TI if one is going for GPU performance. They're not really comparable builds.
CPU: 2 x Intel Xeon E5-2637v4, 3.5GHz (4-Core, dual threaded, HT, 15MB Cache, 135W) 14nm
RAM: 128GB (8 x 16GB DDR4-2400 ECC Registered 2R 1.2V DIMMs)
Graphics: NVIDIA Quadro P4000 GPU, 8GB GDDR5, 105W, Single-Width, PCIe 3.0 x16,
Seagate 1TB Exos 7E2 HDD (6 Gb/s, 7.2K RPM, 128MB Cache, 512n) 3.5-in SATA
just a quick build i did over at Silicon Mechanics