The Btrfs feature updates in Linux 4.8 has the big ENOSPC rework as well as other clean-ups and improvements.
From the Intel Xeon E3-1280 v5 system with the four OCZ Trion 150 120GB SSDs I compared the Linux 4.8 Git numbers to Linux 4.7.0 stable. Tests were done with a single disk and then RAID0 4-disk, RAID1 4-disk, and RAID10 4-disk. The default Btrfs mount options and other defaults were maintained.
Sequential writes appear faster with the tested RAID levels across the four disks when using Linux 4.8.
For many tests there wasn't a change in performance with Linux 4.8.
The PostgreSQL database benchmarks are a bit worrying though that it appears the Btrfs performance does regress under Linux 4.8, even when using a single disk. I'll be back with more Linux 4.8 file-system tests shortly, but aside from the possible PostgreSQL regression, there weren't many clear performance changes of these initial Btrfs tests on Linux 4.8.