The engineers at D3 went straight to the engine compartment and first
came up with a new air intake system to increase HP and increase the
efficiency of the motor. The D3 Air Intake System replaces the OEM air
intake and uses the same mounting points and hardware. This modification
is 100% bolt and requires standard hand tools to install. Next D3
addressed the suspension of the ATS. 
The D3 Street and Competition Lowering Springs will lower the vehicle, as well as, increase the handling to ensure a quality ride and reduce body roll. The springs will work with the OEM suspension and will keep the technology that Cadillac has developed intact. D3 has meticulously designed their lowering springs to maintain the feel of a Cadillac, but improve the handling to increase its turning capabilities.
For a more aggressive suspension solution D3 has developed a coilover system for the ATS. The D3 Coilover System will fit the ATS without having to fabricate or change any install points or hardware, providing a OEM quality fit and performance. The coilover system is an adjustable system that can be lowered and have any spring rate that the driver desires. This set up lets the driver choose how the coilover system will work according to each driving situation, whether on the track or just on the street. The adjustability gives the option to have set the vehicle up in just a matter of minutes with the provided tools, to make adjustments just about anywhere.
article and images source: Cadillac