I got 15,865 seconds in Wprime 2.06 with a 2,5 Ghz i5 Sandy Bridge. Something looks odd with your result. Do the following:
- flash Bios F11a
- disable Turbo mode
- do not overclock the CPU
After that we can be (more or less) sure your sample is running with exactly 2,0 Ghz. Then repeat WPrime and I can recheck with my 2,0 Ghz clocked Sandy Bridge.