With the GPU denoising being available now, the Denoise Amount spinner should be available in the VFB during IR. Right now it's always greyed out.
At the same time, the Denoise Amount parameter in the renderer dialog is available during IR, but it has no effect during IR or after stopping IR, so it's not possible to see the render result without any denoising. It should be made to work with IR too, just like it does for normal offline rendering.