First of all, 40PSI is not optimal. Thats WAY too high. Look on the door frame of your car, the optimal PSI is 29, at 40PSI any tire is going to ride like crap on the ES. I air mine to 31 and it rides great and gets great economy really.
Again...read what I am saying.
you are not getting more responsive feel and you cannot run lower pressures to get the same treadwear when using nitrogen vs using air
Read that at least 3 times...it doesn't seem to be getting through.
Your tires will wear slower at a higher PSI, but 40PSI is probably so high that you're getting uneven wear in the center of the tread.
Personally I bought a Lexus for the smooth quiet ride, so if I sacrifice a little treadwear by running the tires at the pressure Lexus specifies for optimum performance its worth it to me.
In terms of treadwear, I personally don't think a good smooth quiet tire is going to last 50k on this car...best I've done is 35k and thats the Michelins.