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!
Your Mileage WILL Vary
Might put your window capture advance? volts to see die?