I expect to be driving mine over dirt roads in about 2.5 weeks. I'll let you know my opinion then. (Regarding your installation question - - I think there has been a thread on this forum about the install.)
I didn't want a spoiler, but couldn't find one without it. I am hoping that the silver lining to having had to buy one will be that it really works on dust. My RX300 had a spoiler, and still got dusty, but the rear shape is so different that I am hopeful this one will work.