It's good to always check the
DS first.
In this case section 2.4.5. Digital Specification specifies: Max SPI_CLOCK is 10Mhz.
I started development with 8mhz and eventually decided to clock it down to 4mhz. I felt it was safer and I have no reason to change this now, regardless of CPU (it is not the limiting factor!).
MoteinoM0/SAMD21 has USB speed serial (baud is irrelevant), which is way different than FTDI - which emulates a UART serial - this requires a baud and can go up to 2mbps. We use 115200 but you can always try 1mbps if you'd like, I believe it should work (nice even fractional clock based on the MCU speed) of as long as both sides play nice with that fast clock (assuming Pi should also handle higher UART clocks).
Last time I tried an M0 serial on the Pi's USB, it locks up as soon as you try to send data to the M0. I have to investigate further - I did find some other online links that talked about that. Maybe the latest raspbian behaves different. The fallback would be GPIO serial instead of USB serial, I would certainly believe the M0 can handle 1 or even 2mbps.