To put things into perspective a bit, as I'm doing a similar thing.
I've got 2 drive bender pools living on different machines. When I copy (sync, allway sync, similar to free sync), I push my network to its theoretical limit. This causes the drive bender console to show between 100MB/s - 140MB/s; I guess its buffering technique causes it to sometimes peak and sometimes fall off a bit.
This sometimes causes even a simple music playback on a network client to stutter.
But locally, I can play blurays just fine. In fact, when I had a vm running with an extremely heavy database spread over all 5 disks in the drive bender pool, I was seeing nearly 200MB/s simultaneous read/write actions. I've peaked either one or the other higher than that, but never had this simultaneous so that was exceptional for me. Playback never suffered even though everything lives on the same pool.
So with regards to backups and syncs, I run 2 backups from my main workstation to my server/htpc, full image on sundays, daily incrementals at 18:00. A separate file backup at 19:00, same scheme full on sundays, incrementals on the rest. Allway sync is keeping photos and music folders in sync on the main pc and server/htpc and locally, the server/htpc syncs the music and photo folder to a backup disk with separate retention settings for modified and deleted files (this is why I like allway sync so much!). Also, locally an image backup is made the same as from my main pc. Last but not least, all library backups are synced to an online storage thingy, Skydrive.
The times at which the image/file backups run are times that we are having dinner/watching the news. No streaming will happen. The syncs from allway sync only have to keep things in sync, so their data copy is typically very low, except when I've done massive changes, and then there's no streaming because I'm working on making changes
. But if someone would want to watch something, it shouldn't be a problem since storage is local to the htpc.
I guess its a plus that I'm having my storage locally in my htpc, it will almost always have enough resources to play. I've had trouble, when all cpu resources were being consumed by some other multicore process like parity calculations for instance.
Goodsync looks nice too, I might have a look at it because throughput throttling is one thing Allway Sync doesn't have.