Check out my Benchmarks.
The whole point in Geekbench is that it measures the hardware, independent of the OS. That's why it's available on Windows and Mac on the same scoring basis. I'm no expert but I don't feel this is informative.
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I have a question for the original poster and others who have done this. You stated that you upgraded to ML with your 2008 MBP. Ever since I installed Lion on mine it has been significantly slower. I have read other threads which stated that the OS upgrade to ML seemed to increase performance in one way or another. Was that your experience (wanting to upgrade HDD aside)?.
On my 2007 MBP, I found it ran better with ML compared to Lion. I would recommend it.