I make few more test and solve a part of this problem , my Graphic card was overcloked with 197.75
I'm return to the stock clocks for this test.
Details for 257.15 beta drivers :
Directlighting : 9.89 Megasamples/sec (variations : 9.86~9.89)
Pathtracing : 3.20 Megasamples/sec (3.17~3.20)
Benckmark render : 12:08 min (default setting , 4000samples/px)
Details for 197.75 drivers :
Directlighting : 11.20 Megasamples/sec (variations : 10.85~11.20)
Pathtracing : 3.30 Megasamples/sec (3.26~3.30)
Benckmark render : 11:50 min (default setting , 4000samples/px)
I don't notice difference in the final render quality .
Regards
Edit : Maybe the final release of 257 will fix this difference of performance.