My knowlage is under average but I don't think this stick can do much more. Pushing it that hard, with all your equipment (hub, ext cooling), I'm sure u are equipped enough to measure somehow ur temps, because as I undestood it from last few posts, it matters alot. Maybe even with ext fan its too hot (or both of them). I am just guessing. But if you are complaining because you feel cheated somehow because you expected 5.8-5.9 I have no answer to that. Max I saw was about 5.80 on one post showing "simulation" or something like that. I am getting 4.40 @796, one piece, no hub, in 3.0 laptop port and I don't feel cheated btw., but respect everyones questions and feelings.
No, don't feel cheated at all, that's just the most I am able to get out of them. I don't think any cooling problem or I should start seeing excessive hardware errors.
If your not running the stock setup I would really check the small heatsink temp with a heatgun before continuing to run them at max clocks. The heatsink is designed for induction flow with a fan blasting air on it in close proximity.
Overheated ASIC will NOT show excessive errors, either the buck controller will blow, or your going to burn out the ASIC. There is over-current and over-temp protection on the buck, so you will most likely kill your ASIC before you see anything wrong.