Latitude 6510 cpu upgrade?

​I have an I5-560m Arrandale with a pga988 socket. It has the built-in graphics. Though there is a long list of Arrandale cpus on the Intel site from Celeron to I7, it is hard to see any performance improvement based just on processor base speed. I am focusing on the I7-640M as maybe the best one ($20 on Ebay). Is there any other performance comparison among these to choose the best one? CPU is not soldered, it has the release screw and is very easy to get to. Thanks! ​

​PS: I do see some E6510s sporting I7-940QM cpus but they must have a different MB with separate graphics. ​

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​https://ark.intel.com/content/www/us/en/ark/products/codename/32724/arrandale.html​

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