Here are my results with risers:
ASRock H81 BTC mobo with 4gb of ram on Win7 x64= only 5 EVGA FTW 750ti cards run stable
ASRock H81 BTC mobo with 8gb of ram on Win7 x64= all 6 EVGA FTW 750ti cards run stable
I get consistent 295-300khs on all cards with +20 core and +410 mem. I think if I go too much more than that it crashes. Since they're FTW cards, that comes out to 1209(1288 boost) clock and 1555 mem. I can probably go higher if tweaking them separately since I probably have at least 1 card that's a weak overclocker, but that is stable so I left it. I'm thinking I will get more without risers, but I have no way to test more than 1 on this board since it's only got 1 16x slot. **Just an FYI, but GPU-Z is saying boost is 1288 but Precision X is actually reporting 1330-1350 for clocks**
On my other rig with Gigabyte 750ti's, I should have researched more instead of jumping on the cheapest board simply because it came with a freebie mpcie ssd, but that one seems to experience the degradation people are talking about when adding more cards.
That mobo is an ECS Z77H2-A4 with a Celeron G1620. Adding 4 cards is fine, but it seems to choose PCI-E 1.1 1x mode for every card on the 1x slots and only PCI-E 2.0 for the one plugged in to the first x16 slot. It boots with the 5th plugged in but only detects 3 cards then. I've tried the x16 slot with x1 and x16 risers and it still only did PCI-E 2.0, which is the only card getting full speed. ***I just checked the processor specs while I was typing this and realized the problem.... "PCI Express Revision 2.0 / Supports up to 1x16, 2x8, 1x8 & 2x4"
I wonder how many other people cheaped out on processors and are getting similar declines in hashrate or incompatibilities with multiple cards. I surely can't be the only one. The ASRock board is running a socket 1150 i3 4130, so I'm guessing that's why I don't see a decline in khs. (Only 1155 cpus i5 and up supported PCIE v3)
I have an ASRock 970 Extreme 4 and some shit 170u CPUs. I have 4x Zotac 750Tis in my rigs and they don't slow down except for thermal slow downs. But then again my cards average around 280-295 to begin with.
So if that's the case then the issues of slow hashrates using pcie/usb riser depends on the hardware. So make sure your processor supports PCI-E ver. 3.0 but of course more hardware testing should support this claim. One thing if youre motherboard supports the PCI-E 3.0 then a graphics card supporting that bus speeds should be use when using risers.
Tech Specs: quick comparison)
PCI-E 2.0 5.0GT/s (raw bitrate) - 500MB/s (bandwidth per lane direction)
PCI-E 3.0 8.0GT/s (raw bitrate) - 1Gb/s (bandwidth per lane direction)
Have fun & join the boat ^^
