I think there are uPNP/DLNA "sniffer" programs that can tell you what the network traffic is and what data is being sent by what device. Alternatively you could try Twonky support to ask if they send bitrate and ratings via DLNA.
If importing the files directly from the NAS into JRiver actually works for you then just use that way and turn off Twonky, as you can then avoid the limited Twonky navigation tree and you can harness the full power of JRiver, setting up your own custom browsing structures and categorisation. Connecting JRiver to a remote DLNA server is probably adding an extra network layer that you don't really need.
I would suggest that the perceived better responsiveness via DLNA is odd - this would only happen if your NAS is more powerful than your PC. A local JRiver library should be lightning fast. Can you cxplain which bits are not as responsive for you in a local JRiver library - is it browsing the library, is it the time taken from selecting a file to it actually playing, is it responsiveness of the transport controls, etc?