First question, what is your max DL speed with your computer ?
I would guess your NAS doesn't have the CPU power required for sabnzbd, as it's a newsgrabber which require some CPU power.
With my Ultra4, with a 2MB/s connection, sabnzbd downloads at around 1.8MB/s, but it's not due to the NAS, the overheads of newsgrabbing saturate my upload capacity.
ReadyNAS Ultra 4 (with 2GB RAM) with 1x WDC WD30EZRX (Caviar Green 3TB) and 3x Seagate ST3000DM001 (Barracuda 3TB) - 8326 GB
Dual switch Netgear GS605 : one main switch in the wiring closet, one for the "distribution" on DLNA devices (TV, Amp, media player)
NAS and internet box/router (Freebox V6) on CyberPower DX600E ASI (usb connected)