I think this came up in a similar thread about a year ago. I had the same problem at a few locations - heavily loaded (but under max rating) units (Lutrons in my case) get hot or very warm to the touch. I researched it with Lutron and finally came to the conclusion that they just get hot, plain and simple, with anything over 400W, regardless if the unit is rated for 600W or 1000W.
A stainless plate actually helps disapate heat better, but transferred the heat to the clients mits, making the problem more noticable.
Sorry to you and your client, but until they come out with a product that keeps the heat away from the rocker switch, it's something we have to live with.
