When getting the testing underway with FS-Mark, when using 1000 files of 1MB size, the performance of Btrfs actually dropped with the Linux 3.4. Btrfs on the Linux 3.4 kernel for this particular test on a high-end Intel SSD had regressed compared to the earlier kernels. With EXT4 and XFS was a performance regression when moving from Linux 3.2 to Linux 3.3 for this Intel hardware.
The performance of FS-Mark when dealing with multiple threads were roughly the same between kernels but overall there looks like there may be a minor performance regression across all three file-systems with the Linux 3.4 kernel. The fastest file-system overall for the FS-Mark 4000 files, 32 sub directories, 1MB size test was XFS while EXT4 was right behind and the next-generation Btrfs was the slowest.
Dbench was the fastest on EXT4 while the performance of Btrfs and XFS were close to one another but noticeably slower than EXT4.