Well boys and girls, I've made some progress.
Currently here is my setup:
2x Zotac Reference 750 Ti
4x ASUS OC'd+PCIe-6pin 750 Ti
Intel Core-i5 4670k
4GB DDR3 1333MHz
6x x1 USB3 Riser Cable Asemblies
Old Seagate 320GB HDD
I have identified two separate issues.
#1 PCIe x1-mode performance for Cudaminer
Cudaminer apparently has a high PCIe bandwidth utilization...thus when going from x16 mode to anything lower, as you get closer to x1 mode (risers or not), you're going to see degradation of performance (10-30 kH/s for 750 Ti in my experience). Cudaminer author is apparently going to be working to fix this ASAP since he'll soon be operating a 750 Ti rig with riser assemblies.
#2 Windows 8.1x64 + > 2 GPUs = scaling disaster: With six GPU's loaded, I could not get > 1400 kH/s. Seeing as how there has been anecdotal evidence of others having success at scaling GPUs in windows, this was perplexing to me. Additionally, the system felt realllly unstable with all of the cards at stock speeds...seemed like just having six cards running at once was asking too much for W8.1x64+Nvidia driver. I tried Ubuntu 12.04 x64...that seemed to scale OK, as I could get ~1600 kH/s, but I was stock at ~stock performance. I learned how to increase the MEM CLK with Kepler BIOS Tweaker, and that helped a little bit and was what allowed me to hit 1600 kH/s. Main issue with a Linux solution is abysmal OC capabilities. Ain't nobody got time for that. Finally it hit me...these guys on these forums aren't paying for Windows licenses, they're torenting Windows 7 off of BT. They, unlike me, don't have readily available MSFT product keys to throw away. That is what bit me in the ass. I installed WIndows 7 x64...sure enough...GPU's scaled, system is buttery stable, and now I could individually tune my GPUs with individual over clock settings ala MSI AB. This is how I now achieve ~1760 kH/s on six cards.
I've accepted that my investment in ASUS 750 Ti's was a bad choice, and I'm dealing with it. The MEM CLK's wont OC well at all compared to the Zotac Cards. Cooling is not an issue nor is availability of power.