How long are your pads lasting?
20,000 miles sounds low but this can depend on your driving style and type of driving, constant stop start driving in town will cause faster wear than country driving.
Is the car garaged as if left outside every time the discs get wet they will rust removing another layer of metal when next used.
Is there a visible wear lip on the outer edges of the discs to confirm the claimed wear or is this just a figure from a Lexus dealer regarding the thickness.
As a comparison my LS400 discs are all originals after nearly 90,000 miles and 22 years.