Roughly its like this:
high leakage = very good clock scaling with extreme cooling and usually good scaling on high end air or water cooling
retail cards have default low voltage to combat high leakage which equals heat
low leakage = average clock scaling (low max. clock card will sustain) but very good thermal properties
retail card will have higher default voltage
Now a lot of people think why low leakage have higher volts and not lower as high leakage. Simple answer is they would not work stable at lower voltages due to how transistors work.
High leakage transistors have shorter (smaller) gate. This allows faster switching speeds at a given voltage but introduces more leakage at a given manufacturing process.
Low leakage transistors have longer (thicker) gate. Opposite to above mentioned.
Process variation / tweaks / etc result in varied gate thickness and hence varied ASIC "quality".
Sorry for oversimplification and I welcome any corrections.
Example:
Record breaking Phenom II TWKR Edition was a product not qualified for sale in retail channel as it was out of spec. in regards to leakage. AMD sent these 'bad' samples to selected overclockers to use them with LN2 and LHe cooling. They were average clockers on AIR or Water due to excessive heat they generated (retail Phenoms clocked better) but given sub-zero temperatures this samples were best of the best at a time.
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