Cont' on my tests on external eGPU:
Just test with the riser 1x by 50 cm USB 3.0 cable (about 10€) : it works well (test with a titan and also a 580gtx).
Edit : for the chinese riser extend to 35cm : it finally not work very well on heavy load. So stuck back to 18cm.
I will try to find something 4x riser (for amfeltec adapter) that go up to 50cm.