I think it varies from dealer to dealer, mine has been very good, new engine, BUT I think all dealers have been told to tighten up on warranty claims on modded cars that would relate to a problem.
It is hard for a dealer to spot a remapped car, though they owuld know for sure if they drove it, that is a sure way of finding out according to some of the technicians at my dealer.
To be 100% of a warranty claim get a switch, or your remapper to switch the map off.
I know for sure if you have a Revo map Audi main network, (HQ) when they look at the car do see some wierd logs that makes them ask, even when the map has been swtiched back to stock, as for other manufacturer's maps I do not know, but once you map your car with a Revo map it does change underlying settings in th ECU that are not switched back to stock, This I know for sure 100%