I was told similar when I had a recent service at an independent, who i have used for years. It was the front lower wishbones bushes and the parts were £460 and his computer was telling him that it was a big job, labour wise. He didn't believe the amount of hours needed so suggested i called Lexus to ask what they charge. I spoke to two separate dealers.
One said it was a 4hr job (at £110 an hour) but seemed to be too busy to talk for too long!.
The other said that he has worked for lexus for years and has never changed the front wishbones on an RX400. He immediately knew what I was talking about and said that the bushes 'were probably like that after about a year from new' and there was no need to replace the wishbone, it wouldn't fail an MOT and that there was no safety issue at all. He clearly seemed to know his stuff and knew exactly what I was talking about. It seemed that he had been asked this a number of times.
I told my garage who were fine and suggested that Lexus dealers know their cars so leave it and see what happens at the MOT.
Your advisories seem to suggest that although the bushes have deteriorated, there is no resulting problem? Which is basically what the Lexus dealer said.
Hope that is of some help.
Cheers