There are a few reasons why your 8-core server could be slower:
1. Your benchmark may not be well-threaded and the 3.0 GHz B95 has a large clock speed advantage over any 8-core Opterons (which run at 2.0-2.4 GHz). However, you'd expect times of more like 45 seconds if that was the case.
2. The power management is not set up properly and the 8-core units are running at their idle speed of 800 MHz instead of at their full 2.0-2.4 GHz speed. That would suggest a run time somewhere in the 120-second range instead of >200 seconds.
3. You are having software issues with the OS the 8-core machine is running on that results in you using a software stack that is by default far a lot slower than you did with the Phenom II.
The 8-core units are basically two of the Phenom IIs sitting next to each other underneath the heatspreader of the CPU. They should perform roughly similarly clock-for-clock and core-for-core.