I'm interested in the notion that it might be a stress problem - why does your vet think this? Because he (your cat) is a pure-breed???
If plastic-licking *is* down to stress, then my new cat has, apparently, been stressed since he was first born, pretty much! The RSPCA know his full history and plastic-licking is well documented in the notes from his previous owner. Yes, he has recently had a change in circumstances and 10 days in a cattery before coming to live with us, but to say he is stressed . . . . well, he is currently lying splurged out on his back, purring fit to bust, right next to our existing cat whom he hadn't even met before Tuesday!!! He doesn't *seem* stressed to me . .. . . just a little bit bonkers!
As an aside, these two new cats seem to be *obsessed* with the bath.
They will happily sleep in it during the day when it is empty but both of them (one yesterday, one today) have jumped into it when someone has been in there, taking a bath! Both times, they have just stood in the water - and it's not as if they didn't realise there was water in the bath; the taps were running! I was more worried they might get scalded. After yesterday, we kept the bathroom door closed but we found out today that they know how to open doors . . . . .
. . . so it looks like we are going to have to start locking the door before turning on the taps!