Most IIMs are strict about the minimum 8 CG criteria with IIM Bangalore being the most rigid. To stay on the safe side, one must maintain an 8.5+CG. However, when combined with other parameters like work experience and gender diversity, even a lower CG of around 8 is considered decent.
So, we can say that an 8.5 CGPA is definitely a good CGPA for IIM.