When Postgres switched from ARC to 2Q in 8.0.2 due to patent concerns testing showed that "there is little if any performance loss from doing this."

http://www.varlena.com/GeneralBits/96.php

https://github.com/postgres/postgres/commit/4e8af8d27315c4f3...

Edit: this ARC paper has a performance comparison of various cache algorithms: http://dbs.uni-leipzig.de/file/ARC.pdf