Hi Neil, welcome to the club, i have only been a 450 driver for 3 months and loving it!
1) that pretty good going at 34mpg, thats exellent as i can only get around 26mpg... i think it is either i am driving too fast or there something wrong with the timing of the car!!
2)chinese? what do you mean... your sat nav is in chinese? shouldnt be... there is no chinese options, although i have also been checking in the far east for the software and no luck..
3)Check your tyre pressures mate... or take it back to lexus - i hope you got it from a dealer...
4) cant help you there on the voice... i dont use it because it too frustrating to get the right command sometimes, even for a southern accent... i suppose you need to have the Japlish to make it understand... i gave up... fastr doing it manually..