The upper limits vary.
All ASIC may be created equal but the package (heat spreader) and the heat sink vary radically.
I inspected my Artic Coolers and one disassembled itself in my hands.
The others were far ftom flat where it matters.
It is one reason I switched to waterblocks. (main reason is/was fun factor)
The top of the ASIC heatspreader(the part you see with heatsink removed) is not flat.
Thermal compound helps mitigate but not always enough.
I suspect some poor performing die are under dips in the heatspreader.
I will deck and lap my ASICs this week and report if anything improves.
(do not try this at home OR at work, risk is high for contamination)
Fastest die I have with Oct ASIC's is 900 but not all die on ASIC will do that speed.
Enjoy what ya have!
![Smiley](https://bitcointalk.org/Smileys/default/smiley.gif)
Your Mileage WILL Vary
![Smiley](https://bitcointalk.org/Smileys/default/smiley.gif)
Might put your window capture advance? volts to see die?