I'd fight them. In CA, they're not required to use OEM parts, however they are required to use parts that are of equal performance, fit and finish of the original parts. You might talk to some body shops about the specific part they intend to use (maybe cold call them and say you're going to replace the stock bumper you damaged by hitting your sisters car or some similar story). If the body shop employees tell you the parts aren't going to be as good, tell the insurer you're going to require they use OEM parts unless they can prove their parts are as good as OEM (in the face of the body shop employees saying they are not), and then let them know that you're also going to pursue diminished value costs.

Insurance info from insurance.ca.gov: http://www.insurance.ca.gov/0250-ins...ment-parts.cfm

Diminished value: http://www.ican2000.com/dvfaqs.html (I'm not saying to go through this site for their services, they just had decent information and was the first link on Google )