The speed stated for networks is the maximum number of bits (not just the payload which in this case includes the audio packet themselves), including any encryption, addressing, housekeeping, headers/footers, signalling and redundancy/error correction bits. It is also stated assuming perfect conditions and that no other networks are present.
So in any network, there is a large overhead of signalling traffic not actually related to the audio packets. The actual audio packet part, say 4.39Mbit/sec, is only a small part of the whole signal.
The payload (i.e. user data) is a lot less than the signal as a whole.
Also the links never get anywhere near their maximum capacity in practise as they are affected by interference (from microwaves, baby monitors and others), other networks in proximity, signal attenuation, and of course other equipment connected to the same network.
To help deal with these interferences, wi-fi networks use dynamic data rate scaling that directly trades error correction for bandwidth, the bandwidth directly affects the data-rate that can be transmitted.
In the real world the data rate you get is thus quite variable and less (often much less) than the maximum theoretical.
Another consideration is that UPnP servers tend to send their data in progressive download chunks (that the NP30/Stream Magic 6 then buffers and re-clocks). As the server is not streaming all the time it does not utilise the maximum bandwidth of the transmission channel.
This behaviour also varies from server to server.
So, although theory says that streaming high resolution audio wirelessly should be possible, when we advise using a wired Ethernet connection for streaming any 24-bit content, this is based on our own real world experience of what actually works.
For pretty much anybody, compressed formats such as MP3, AAC, WMA and Vorbis should stream without problem over wi-fi. Lossless/uncompressed 16-bit FLAC/WAV also.
For 'better than CD' high resolution content over wi-fi we find 24-bit 44.1/48k FLAC is usually fine, 24-bit 44.1/48k WAV is also often fine (but not always), 24/96 FLAC is sometimes ok and 24/96 WAV often not.
This is not a hard and fast rule. It does depend on your equipment, distances and what other networks are present where you live.
So, as we seek to give good advice and set our users expectations correctly we simply advise that for most people: 16-bit content use wi-fi or Ethernet. 24-bit content, use Ethernet