The goal of iray's matte objects is to make it appear as if the CG objects and matte objects are part of the same scene, i.e. making that faucet look like it's actually on that ground. The faucet will therefore always reflect the ground.
At first glance, it seems to me that what you want is to composite two different layers, a faucet render and a shadow render, without either having any effect whatsoever on the other. That's something that classical matte workflows do provide, but not something that iray provides: iray embeds matte objects within its path tracing logic, rather than implementing matte objects as some sort of compositing operation.
But there's one problem I can solve: that of the background color affecting the ground color. What you really want is to replace the background color by white (or red), without affecting the rendered result. That's OK, but it should be done in Photoshop as a post-render step because everything you provide to iray will affect the render. The background is used by iray as the matte color... You need to set the "Render for Compositing" flag, in the iray matte environment shader, and avoid overriding the background color entirely. This will cause the background to render black, without affecting the color of the ground. You can then use Photoshop to composite the white background at post-render, using the alpha channel.
Does this help?