Ok, little update -- I bit the bullet and went SSD... but not just any SSD.
I could not justify the cost of the write increase over the RAID on a SATA SSD. Couldn't do it. $170 a tb is nuts for barely a hair over twice the read time of a single HDD (where at 6tb $30/tb) it's even if the writes are spectacularly improved.
However, 14x the speed of a HDD? I'm willing to go to $300/tb for that... so I did. M.2 PCIe x4.
Now, I'm on a Asrock Z87 Fatal1ty Killer motherboard, which doesn't even officially support NVME, without which you can't boot from it. The board is six years old and NVME is bleeding edge.
... but their beta drivers DO theoretically add support, so I decided to try it. Got a cheap Syba M.2 to PCIe x4 converter and a 500gb WD Black NVME 3D drive. Used AKOMEI to copy the existing boot partition from the decade old WD Caviar Black HDD (hey, black to black!) and sure enough, boots right up... to the point that I think windows now starts up in less time than the bloody EUFI firmware does in dicking around. From end of BIOS to the login page I doubt it takes more than three seconds.
... and the numbers? Well, not what is advertised or the theoretical limits, but I'll take it.

Yeah... SATA can go suck an egg. THIS is what it takes for me to embrace SSD's as a technology.
Though honestly we've been hobbled by ATA's legacy -- land's sake at the heart of it the core protocol is still built on the concepts of the 16 bit AT / ISA Bus with nothing more than kludge upon kludge and hack upon hack layered atop it.
Sometimes you just need a clean break in technology.
Otherwise I'm just sitting here with my thumb up my backside waiting on the HDD's since they (NewEgg) could only ship ONE of them to me due to low stock levels. :/
That said, I really wasn't expecting the parts to arrive TODAY since I ordered them around 1PM yesterday. That's service.