Hi,
I did it when I bought my ISF last year. There were no signs of anything amiss, but given the issue appears to occur within the radiator and can fail without any visible issue on the outside, I didn't want to run the risk. From what I can glean, it's not that common, but I decided it wasn't worth the risk for the sake of a few hundred quid on parts.
I did mine DIY as I like fettling and it reduces costs! There are guides online and it doesn't require any specialist tools. A decent indie garage could carry out the work without going near Lexus main dealers.
It's like anything. You pay your money, you take your chances. I wasn't prepared to risk a leak developing and ruining the transmission for the sake of a reasonable outlay and a few hours time.
Ian