No, these old work boxs are only rated for about 6 pounds max. So, you can remove the old work box from the wall and then install a metal box that has a bracket that is pressure screwed into the studs on either side of the box. These are normally used for ceiling fan installations but are also good for heavier wall fixtures. Make sure you buy the type that is screwed in by hand and not the type that needs to have screws drilled into the studs to attach the bracket. Below is another link showing you what they look like
http://www.thisoldhouse.com/toh/prin...767799,00.html
Here's even a better picture of what the remodeling type fan brace looks like
http://www.westinghouseceilingfans.com/ac_fanbrace.html