For the SoundBridge FLAC is transcoded to WAV by the server (typically Firefly). So the data rate transfered is 1.4Mbits/sec. This is well within the capabilities of the hardware but is a much higher rate than you would see using MP3 files, for example.
However, if the WiFi connection is operating on the margins of acceptable signal strength then trying to transfer this many bits may in some cases give rise to increased rebuffering. Solutions include, relocating SoundBridge and/or router; or use a wireless bridge device (which will likely have better WiFi performance than the SoundBridge); or use power-line networking; etc. I regularly stream 1.4Mbits (and even 1.5Mbits) to my SoundBridges without problems using the built-in WiFi, so it is definitely possible.
Roku M1000, M1001, M2000, R1000, Roberts WM-201, Stream 83i