It looks like your factory wheels' offset is +50, so the +40s you are interested in may not work. factory widths are 9.0 (front) and 9.5 (rear)
Tire upsize would be 225/35 - 20 and 265/35 - 20, but then the wheels would likely be heavier, meaning slower acceleration and longer braking distance.
See:
Wheels for 2008 Lexus IS F (tirerack.com)