Interesting the LEPA 1600 gave you problems. They're sold out everywhere for people making 5x280x rigs.
The rig was failing all the time (rebooting) as soon as the fans started to go over 70%.
After a lot of trials, we went to the conclusion that if we use 1x powered riser connected to a molex then it will always fail.
We tried with 2 Lepa 1600 and we had the same problem. I replaced by a Corsair, and all went well.
I'm using the Lepa with 16x non powered risers and all is fine.
I may have done something wrong, but my conclusion is that the Lepa 1600W cannot bring enough amp on its 5v line (I know, sounds dubious)
Eric
So in all of your tests, you may have been using a faulty (arguably DOA, if new) PSU. It's hard to draw any valid conclusions from this if n=1