Alright I am in the insurance industry so let me offer a little insight.
You are collecting under your own policy and thus are subject to the limits of your policy. in your case you opted for $30 a day rental coverage and thus you can be reimbursed up to that limit regardless of style vehicle. If that does not afford the BMW rental i'm sorry. look up some rates for those classes of car and ask your agent how much it will cost to get those limits and then make your own choices for the future.
Your other option is to go back to the other party and explain whats going on, tell them you want to get a comparable vehicle to the one you are driving and ask them if they will cover it (specifically say your looking to rent say a bmw not go by class). if they approve it then your golden and just wait for the reimbursement.
As for grounds of going to court over this and doing more then wasting more time and money? I would probably buy some scratch offs first.
In summery, always read your insurance contract and when in doubt, ask your agent.