Hi , the fitting of 20 inch wheels could have a warranty implication if they want to make things really difficult , but , To answer your question fitting lowering springs will definitely void the shock absorber warranty but this really isn't important as the standard shocks are poor any way .
Fitting the Sport kit "Shocks and springs" will come with its own warranty , which is probably longer than the VW warranty any way and they can not void the warranty on anything else in the car if a shock and spring kit is fitted. The truth is the KONI shock will be a lot better for the Chassis as it absorbs the bumps in a more controlled way. Sensible drops of 40 mm wont flag issues with Drive shafts either .