My Old Kia was remapped, never had any issues with warranty
Most "boxes" im their simplest fool the ecu into thinking that the engine is running cooler than it actually is, so pumps more fuel into the cyclinders to boost power, a combination of increasing the boost pressure makes the car faster.
Personally i wouldn't use any of the "boxes" i'd go for a proper remap, more expensive but in my opinion worth the investment,
I wouldn't worry about any warranty problems Kia's are fairly bulletproof