Is this an OEM exhaust ? If so, you can get the OEM hanger donuts from Rob at Needswings, there should be no issue with them coming off.
If it is aftermarket cat-back, then there has been some mention on the forum that the hangers don't have enough curve in them, I think this is in regards to the MagnaFlow.
I replaced my OEM with an OBX Racing cat-back and have no problems with the hangers.
The OBX is basically build just like the Borla cat-back.