In firmware v1.01b03 libtorrent 0.15.10.0 was used. In new firware 1.02b01 newer libtorrent 0.F.0.0 (probably some proprietary build from 0.16.x.y version of libtorrent) is used behind web interface. However, there's new torrent library behind and one can expect some bug fixes.
However, I suggest installing Transmission client on NAS which has much better web interface and much more features.