1. CP is more than just the exhaust & +15hp. Many threads on this and all subjective!
2. Up to 17K was pre-0%. 10K discount +0% not too bad on factory order now. 15K discount may be achievable if you shop around, however, only likely if you can find a spec changeable build slot and can be registered prior to year end.
It is possible to beat 15K & get +0% on last trading day of Dec if you are in a position to buy a stock car & register on the day.
Another option worth considering is M6GC @ -20K +0%...