I was told to give a full invoice valuation of car and the insurer took ANY original equipment in that price as being standard factory fit and quoted accordingly. I wouldn't say the MFSW is an added.... its in the original specification as per invoice.
I believe the extras that need specifying are high value- easily stolen items like upgraded wheels or after market modifications like aero kit. I'm no expert but the insurers have been really good with me (17 0 NC etc.) and thats what I was told to watch for. The insurers will obviously be more interested in performance enhancements more than anything else.