Ok, so i think i figured it out. Speaking about performance, BTRFS did the best job.
I tried multiple linux based NAS, made all with the same hardware but with different filesystem.
I used FIO (http://freecode.com/projects/fio) to run benchmarks as suggested by a Veeam Software white paper since I'm using their backup product (https://www.veeam.com/wp-veeam-backup-replication-v8-designing-planning-backup-repository-performance.html) and these were the results, using consumer level hard disks (SATA WD RED 5400rpm 64mb cache). I believe this results to be solid and inspiring for further tests, just do some math for pretty self-explanatory percentages.
900GB non-zero generated data:
ZFS pool 2x1TB: bw=27300KB/s, iops=53, runt=3600053msec
BTRFS vol 2x1TB: bw=41541KB/s, iops=81, runt=3600033msec
BTRFS vol 2x1TB*: bw=48223KB/s, iops=94, runt=3600024msec
1800GB non-zero generated data:
ZFS pool 2x1TB: bw=19528KB/s, iops=38, runt=3600077msec
BTRFS vol 2x1TB: bw=31593KB/s, iops=61, runt=3600031msec
BTRFS vol 2x1TB*: bw=40911KB/s, iops=79, runt=3600023msec
5x10GB seq zeroed files with free space < 10%
ZFS pool 2x1TB: 111 MB/s 114 MB/s 112 MB/s 112 MB/s 124 MB/s
BTRFS vol 2x1TB: 155 MB/s 162 MB/s 145 MB/s 126 MB/s 95,4 MB/s
BTRFS vol 2x1TB*: 293 MB/s 296 MB/s 290 MB/s 283 MB/s 281 MB/s (LZO affects)
*Mount options autodefrag, compress=LZO, nodatacow.
**Ext4 and XFS resulted 20% or more inefficient compared even to zfs, that was the minimum entry level for my benchmark.
Please note this is just a contribute to "real life" benchmarking, I'm not stating X is always better than Y, but in my specific case of course it is.