I do not believe that the strut top mounts changed on any of the 2006 Impreza based models so you should be good. Might want to just ask the folks at NASIOC in the suspension forum: Are the strut tops the same for the WRX in 2005 and 2006. If the answer is yes than you are for sure good to go. They never know about the Forester but they know all the WRX details.
Regards,
Justin Wade
2004 FXTi