Get the Whitepaper called Bypassing IDS with Return Oriented Programming. It heavily discusses and shows the point of leveraging polymorphic shellcode in order to bypass detection.
You can download it from the following link: https://packetstormsecurity.com/files/download/105535/rop-ids.txt
Source: https://packetstormsecurity.com/files/105535/Bypassing-IDS-With-Return-Oriented-Programming.html