Hey....

Is there any firmware version higher than 2.11?

If yes, what is the highest and what are the benefits?

And where can you get it, yet? Thanks in advance...
There is at least 2.12, as per this site:
http://www.ereaders.nl/iliad_software_release_2_12/

And a 2.12.1 (maybe only for the Book Edition), as per this site:
http://www.volkerschatz.com/hardware/iliad/iliad.html

Regarding how to get them..
I think the iliads get updated via irexnet. It may just not be possible to update them now.