Well, just to sport some data. I've found Geekbench results for the 13" early 2011 MacBook Pro for the 64-bit test on both OS X 10.10.5 and 10.11 (obviously still on beta). And by the results (despite the fact that the second one has 10GB of RAM vs the one running Yosemite that has 16GB) I wouldn't say El Capitan is that much optimised on this model...
10.10.5: http://browser.primatelabs.com/geekbench3/3207397
10.11: http://browser.primatelabs.com/geekbench3/3222377
I know this has nothing to do with GPU performance but it gives a glimpse at how the beta for El Capitan doesn't seem to be that optimised on this machine so far.
That there is interesting. It feels like it runs smoother overall but battery performance hasn't changed and my fan runs at full all the time playing Modern Warfare where on Yosemite it would cycle between fast and slow, indicating a more efficient processor load.
I wonder if that'll change by the final release.