Many people are guessing the BMW M4 F82's performance rating as the required performance rating. We'll find out the required performance rating for the last race today.
The only way to get free Gold is to do daily assignments. However, the free Gold gained from daily assignments will be a remarkably slow process that it'll take too long to generate enough Gold for transferring performance parts from BMW M4 F82 (Razor) to BMW M4 F82 (stock).
My suggestion is to race against Heidi to get more BMW M4 F82 (stock) blueprints and get the materials required to upgrade the BMW M4 F82 (stock)'s performance parts.