Extended warranties are usually not a good idea financially on a well-built vehicle like a Toyota/Lexus. Take the same money you'd pay for the warranty and put it in an investment account. If you need a transmission 5 years form now, pull out the money. If not you get to pocket the money and use it for something else
I will say the 6-speed in the 460 seems to have more problems than the 5-speed in the 470, at least ancedotally. Quite a few reports of 6-speed problems on there, whereas there are relatively few for the 5-speeds. Still better than a Ford/GM/Chrysler product but they do seem more failure prone.