Here are the bonnie numbers:
btrfs
Sequential Output (Char) 238
Sequential Output (Block) 48884
Sequential Output (Rewrite) 31956
Sequential Input (Char) 4596
Sequential Input (Block) 89337
Random Seeks 114
Again, the sequential output may or may not make sense, but it is consistent with the previous number. The sequential input number is way too high, as my disk shouldn't be able of reading more than 75MB/sec (so says hdparm). I'm starting to lose faith in bonnie...
I tried to rebalance, but I got an oops... So that's not working for now.
I'll play with this for a while before I consider putting it on my NAS. :)