You can see from the performance references for both Android and iOS that FADPCM performs better than Vorbis. You could use Vorbis over FADPCM if compression is too low, or PCM for short sounds if FADPCM cost is too high. In terms of sample rate, a higher sample rate will have better audio quality but at the cost of using more resources. The defaults are just there as a suggestion.
Also, why does streaming a file take so much less CPU than playing compressed in memory?
This is because streams are decoded on the stream thread. You need to compare against the “stream” CPU graph in a recorded profiler session (only available in FMOD Studio 2.02+).