This is a very common problem on the IS200 and is due to the way the suspension geometry is setup. The OEM settings often cause premature tyre wear on the inside edge.
I don't know where you are in the UK exactly, but a trip to Wheels in Motion in Chesham, Bucks, for a geometry setup (which will cost you £85 plus VAT) will sort the problems out and improve handling too. It will be money very well spent, as lots of LOC members will confirm !