ib_logfile opened with O_SYNC when innodb_flush_method=O_DSYNC
Yes, this O_DSYNC (the option) is fundamentally useless because MySQL/InnoDB engineers saw that it was dangerous in certain cases (corruption), and disabled it in many cases, making it a synonym to O_SYNC. it also does an fsync() for the same reason. 

The main thing you want to tune is O_DIRECT vs. default (flushed), and maybe ALL_O_DIRECT on Percona.

In your case, I suppose you have good latency to the filesystem (where O_DIRECT shines). Sometimes, having the transaction logs on the filesystem cache can be hurtful (although such a difference seem a bit too much for me - did you compare it with O_DIRECT, available on all versions of MySQL, which only avoids data file caching?). 

Also be careful, because sysbench may not be a good representative of your real-world load. Response times can vary a lot depending on flush_log_at_trx_commit, I assume you have that as 1.

According to Dimitri, from Oracle-MySQL, the solution upstream comes in 5.7. I like it for now in Percona, as you can choose.