I just switched to Geico today and afterwards ran a quote for adding a 2000 Type R (online). They actually had a section where you could check on the items that are additional or added, such as mag wheels, stereo, etc... and you could put a dollar amount as well. After I submitted it, they informed me that it would take at least 30 minutes for someone to review it. I received the quote and it would add an additional $430 every 6 months.
I think it would be a better thing to disclose your aftermarket items up front rather than try to negotiate and complain to the insurance company afterwards.