First of all, we do not speak of contemporary events or allow 'what if' questions in this subreddit.

Second of all, the Supermarine Spitfire is a 70-year old plane. The reason to why it managed to help in the defeat of Nazi-Germany was the fact that it was technologically advance for its time, just as the P-51 Mustang or any other late WWII plane. While the plane would be used beyond WWII, it didn't last long against jet planes or modern anti-aircraft capabilities.

Would a squadron of Iraqi Spitfires slow down ISIS? We honestly don't know. Since we have no real idea or figures on ISIS weaponry or anti-aircraft capability, it would be hard for anyone to give an answer - even a speculative one.