I've currently (slowly) migrating from Dropbox and BTSync to Syncthing. I'd say you might have a glitch in the matrix if BTSync is running that high without any active transfers.
On my (PowerPC!) NAS, it was constantly using lots of CPU and RAM, although I have a feeling plenty of that was caused by swapping to disk (NAS only has 128MB). I love the fact that BTSync have the forethought to provide a PowerPC client for people with outmoded NAS's like mine, just disappointed it was still too heavy to work reliably.
It's one of the main reasons I picked Duplicity for backup.