The aero is limited to a lightly modified Ben Sopra Circuit Version body kit and that's about it! When other unlimited cars are sprouting huge wings and sophisticated CFD shaped undertrays, the HKS car makes do with a street tuner body kit! The fact that a stock GT-R has pretty good aerodynamics right from the factory helps.
The Ben Sopra kit has a decent splitter that blends with the GT-R's stock nearly flat bottom floor. The middle of the splitter has a kick up that on a race car is there to reduce pitch sensitivity but it's probably more for looks here. HKS added stays to keep the street style splitter from ripping off. Still, it is an appropriate size and shape for a “safe” increase in downforce.
The sides of the splitter have vortex generators that might do something to create a side vortex to prevent air from spilling under the car and to increase the effectiveness of the splitter. The real answer is probably that they look cool!
Canards are rather inefficient at creating downforce and have a lot of drag. They can create useful side vortices however.
However, this huge cutout makes more drag and reduces stagnation pressure for the front splitter greatly reducing its effectiveness. The heat exchangers do create some deceleration of the air to create some pressure differential though so the splitter still does something.
These carbon Craft Square racing mirrors have low drag and minimal frontal area. They probably reduce drag compared to the stock car.
The vortex generators on the sides probably do little back there in the turbulent air behind the rear tires. Here is aero for style!
The center exit exhaust may have an effect that helps the diffuser work better but we bet the stock GT-R diffuser might work better!