Assuming I have a SIP provider which handles both my data and voice traffic through a single router, is QOS still effective once the packet has left my LAN, or is prioritizing data and voice packets differently only important on the LAN?
That is totally dependand on how your ISP does their QoS or even if they do QoS on their network
It depends mainly on your router. (does it do any prioritizing?).
It also depends on the other routers, but those probably have bigger bandwidth then yours.