but can you help me to a site that benchmarked the gpu's with an E5300 at stock clocks(2.6ghz)
You would get less, though the loss would probably be variable. Hardware sites would always test with the latest and greatest cpu to eliminate possible bottlenecks and just see what the gpu could do. In this day, you'd probably find no reputable hardware site that would review a graphics card, compare it to other graphics cards, and then mix it also with the cpu (especially older cpu's). It would just take too much time and effort and the publicity gain would be little.
The closest thing you may be able to compare it to, is the Pentium G9650:http://www.tomshardware.com/reviews/phenom-ii-x2-555,2540-8.html
Or maybe the E6500 (though it is clocked higher):http://www.tomshardware.com/reviews/best-gaming-cpu,2439-5.html
If you're really concerned with bottlenecking, try overclocking your cpu to 3.0~3.3 and judge for yourself if the gain is sufficient.