DRBD 8.4.3 has some great speed improvements. To take advantages of it use the following steps to build and upgrade the ubuntu packages (thanks to icamargos repository).
apt-add-repository ppa:icamargo/drbd
apt-get update
apt-get upgrade drbd8-utils
apt-get install drbd8-module-source
module-assistant auto-install drbd8
after reboot you should see the new version:
root@nas4-1:~# cat /proc/drbd
version: 8.4.3 (api:1/proto:86-101)
Update: Hint for using this module with acutal kernel releases.