"In essence, the presence of an aftermarket part by itself does not "automatically void" your entire vehicle warranty. However, should the improper installation or function of an aftermarket part cause an O.E. part or related system to malfunction or fail, what will likely happen is that the Dealer will refuse to perform a warranty repair or replacement of that part or system. They are entirely within their rights to do so because unjustified component replacements (read: no defect in workmanship or material as per O.E. operating specs) means stiff warranty audit repayments or even warranty fraud charges. Dealers are not authorized to void a warranty (only VWGoA can do so, in writing), although many wrongly interpret refusal to repair under warranty to mean that the warranty is void.
Be careful in interpreting the warranty booklet texts properly. Keep in mind that the manufacturer is obligated to warrant the workmanship, materials and vehicle system functions as per their original design/build specifications, test protocols and US certification compliance, and that the entire vehicle is made up of many interdependent systems. For example: excessive lowering of the vehicle or installing stiffer springs alters the rates at which vehicle weight is transferred during panic braking. Should these rates exceed those calibrated in to the various sensors that provide longitudinal and lateral acceleration data to the Airbag system (and other systems that share such data on the vehicle CAN-Bus), proper airbag deployment could be compromised.
Caveat Emptor. If you want to play, be prepared to pay"