The warranty is only void on parts that are directly affect by the mod. So for example a heatshield would not be replaced on a car with an aftermarket exhaust, but you would have no problems getting your steering wheel replace if it was faulty.
Depends on the dealer with closely related stuff, i had one dealer refuse to replace the headlamp adjuster on the rear axel as the car was lowered, took it to another one and they agreed to do it.