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. 

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