You're not doing anything wrong, in general the viewport is just an approximation of the actual render. As your renders use more and more sophisticated effects, the viewport becomes more and more approximate. That's how it's always been.

If you have the GPU power then you can set the main viewport to Iray rendering and get that accurate feedback you're after. For those of us with weaker GPUs... stick a small 'Auxiliary' viewport in a corner and it will show an Iray render at a size that your GPU can manage.

regards
prixat