Dedicated race car.
For racing on a 1/4 mile track it is doubtful you will ever need rear brake cooling via duct-work.
For autocross racing on tracks I'd still say it is pretty doubtful you will ever need rear brake cooling duct-work. I'm not saying there is no purpose or functionality, only that the level of driving it would require to get to a point where having rear brake duct-work be worth it would be dedicated race car doing professional level driving.
For 99.9% of people, you will never need more than the following setup, which is in my opinion pretty overkill:
2011-14 GT Brembos or 2013 Shelby GT500 front and rear Brembo brake upgrade
2-piece rotors front and rear
Stainless steel brake lines
High-quality high-temp brake fluid
Front brake cooling ducts