Flac files are compressed - its part of the standard. The actual compression rate depends on the track but it averages about 50%. 

So is the issue that my Wiim device isn't decompressing them and is instead playing the compressed version?

I haven’t used the Wiim mini and don’t know what the interface is reporting so I can’t say. However, the audio streaming bitrate if it’s playing lossless hifi tracks should be fixed at 1,411 kbps, the same as a CD. If the Wiim is instead reporting the network data transfer rate then 900 kbps would be about right.

The 50% compression rate I noted is an approximate value for sustained transfer and may be optimistic. Assuming FLAC files are compressed 50%, then a 1,411 stream would compress to 700 kbps or so, then add say 10% for network overhead and you’re close to 800 kbps. If the Wiim is reporting the network traffic bitrate then it looks like the FLAC compression rate is a bit over 55%. If it’s decompressing an mp3 stream I would expect it to report a streaming bitrate of 320 kbps and the data transfer rate to be something less. 

Hopefully someone else has one of these and can clarify what its reporting.  

I did get some feedback on another site that Wiim is reporting the uncompressed rate but the DCA chip is decompressing it. So it would seem the decompression is happening on the back end and it is reporting the compressed rate. This also tracks with the fact any mp3s I play show exactly 320 or 256. 

Lossless codecs like FLAC were invented to encode uncompressed PCM audio like the WAV format from 1411 bits per seconds for a stereo file with 16bit resolution and 44.1khz sample rate to something smaller, but still lossless for storage or transmission purposes, i.e. when being decoded in the player software or hardware back to the same 1411 bits per second.