The positive result is that you will have an update on your case 60 days after filing the mandamus. If your case is within normal time, it will not speed it up. If your case is beyond normal processing time, USCIS and DoS will usually ask for up to 90 days of extension to make a decision on your case which can be favorable or unfavorable. You can adjust your status with DoS approval and doesn’t need to wait for USCIS final approval. You will eventually get the USCIS notice of approval which you can present if you have an interview for AoS which may not be needed.