Just jumping in here - I have a 2009 GS300SE, with 96K on the clock - the previous owner clearly replaced one of the offside rear exhaust parts, and now I am having to replace both cats and middle sections, due to rust - £2,000. Ouch. This is from an independent Toyota/Lexus specialist, not a Lexus main dealer. The mechanic showed me unde the car when it was on the ramp - the flange between the cat and the middle section was badly rusted, on both sides. And the middle section on one side also had a leak, which caused an engine warning light...
So they can rust away at around 10-12 years, yes.
