Hi phil,
About audio quality, I can tell you that’s it is really good.
I can’t give you real SNR value, but I use plenty of daisies on stage for synth, fx and guitar pedals and it is even sometimes quieter than certain pro hardware stuff.
About fft, with a big buffer size, it should be ok.
It is always better to use acceleration framework to speed up things, there’s a subject here but I don’t know if someone achieve to make it work : CMSIS DSP library support (arm_math.h) - #6 by malloyca