Hmm, ok, I'd say also look at your disk stats, mainly around the IOWait time to see if you're being bottlenecked by your disks.
There have been some various improvements since 8.3.3, so you might want to test upgrading to something newer (8.8.1 was just released this morning), but I don't think you'll see anything significant improvement wise (maybe just a few more % increase, if that).