Yes, the i-140 premium processing clock is stopped when USCIS sends an RFE. The 15-day timer is restarted as soon as your employer sends the RFE response and is received by USCIS. Most employers and attorneys use the FedEx overnight service to send the packages to USCIS. USCIS also recognizes the FedEx delivery dates as receipt dates in most cases but you should not assume that the dates will be the same. The final authority is USCIS and they can print a different receipt date than the FedEx delivery date on the i797 notice of receipt.