Ok, so I tested some more GTX 580 with and without IHS. If you make good contact (Freeze or GC-Extreme thin layer, backplate and high pressure) and have a good card without IHS you can do ~1525MHz 3DMark03, ~1450MHz 3DMark11 and <1400MHz Heaven.
In the same condition but with IHS you gain ~125MHz in low intensity benchmarks (3DMark03, 3DMark01, 3DMark05, AQ3,etc) and more in high intensity ones (Vantage, 3DMark11, Unigine). The only card that have IHS is a Matrix that's a poor clocker on air and on LN2, but I managed to do 1520MHz 2k3 and 1375MHz Unigine easy...
The way to go is definately with IHS but I still don't understand why? The question is how do I mount now the IHS back on my very good Lightning?!
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