I have asked a Lexus technician and it looks unlikely a 17inch wheel will fit my 2012 GS450h Premier with 18inch std wheels. The front calipers are very close to the standard wheels. Thanks for your comments everyone.
I wanted the softer ride, but it was me being a bit too fussy! I had done the same with my Mondeo changing from 18in to 16in and the ride difference was remarkable. The ride in the Lexus is afterall very comfortable anyway.