I'm not a real engineer here, but just like your "guts" tell you the issues you have aren't from the extenders, my experience in electronics tell the same.
You have a 5v low amp passing on those extenders just like the whole motherboard circuitry, too low to create a real em interference. You just have to look at the printed circuits mb for the answer, they are not shielded in any way.
The real issues i had was with the PSU's having a common rail for the 12v feeding more than one GPU. The interference created on the rail by every card makes the ones with lower quality filtering stage, capacitors and such, have an extreme varying supply in their actual circuits. All components connected to a common rail induce interference back to it (GPU, HDD, CD-ROM, you name it).
The best solution i could think of was have an independent PSU for every card and the MB too. Using cheap 450w PSU (20 $) for every card solved the internal interference in the setup i have. Off course a little hacking
is involved to trick every PSU it has a load and supply the power. All PSU must share the same ground connection [edit->] on the high voltage side, same 110-220v extender [/edit], take extreme care with that if you get into it some day.
A nice guide
on the "solution" i posted found on the inter-webs.