In most cases you cannot easily port from Ardiuno to SDK (C++ -> C). But you can call most (all?) SDK libs from Arduino. Strandard task strukture in Arduino is simpler (setup, loop), but you can also use SDK tasks in Arduino.
Also, Ivan Grokhotkov, maintainer of the ESP8266 Arduino now works for Espressif.
I did my projects with the (open) SDK, today I would start them with Arduino.