BlueFlashCmd.exe -trans "SPIDEBUG_FILE=spi.log SPIDEBUG=ON" -chipver
The key parameter here is SPIDEBUG_FILE - logging is activated when it's specified, and thus logging is always done to a file. One way to have logging go to a console is to use SPIDEBUG_FILE=con: , but then it won't be possible to redirect it.
Besides SPIDEBUG=ON, there're other logging levels/features:
Looking at the output produced, it turns out it spends a lot in calibrating SPI speed, a way to that down is to use SPISLOW=ON setting right away.