I have plenty of I/Os on the host MCU to control the Si446x. Do I benefit from connecting all the GPIOs of the radio IC to the host MCU? How should I configure the GPIOs?
It depends on your application. You may want to consider configuring a GPIO to CCA (i.e. Clear Channel Assessment: it indicates whether hitting the RSSI threshold, or not), RX/TX State, RX FIFO Almost Full, TX FIFO Almost Empty, CTS, etc. Also, POR at GPIO1 may be a useful configuration for more efficient power up. At room temperature the POR typically takes ~900us-1ms, and the absolute maximum is 5ms. Polling GPIO1 can be more efficient, since you do not need to use a 5ms for every power-up. Note that wiring the SDN pin to the host MCU is highly recommended (there is no SW reset in the API); this also requires a host MCU I/O.