I have, and LOVE, the utopia back rest.
However, a warning...the passenger pillion provides some additional support for its bracket...the steel of the pillion seat base going through to the back of the fender helps provide some stiffness.
if you remove the passenger pillion and run the utopia, you can tear out the mounting bolts where it attaches to the rear fender.
This is known as the "Serk cut" since it was pioneered by my 9 foot tall friend, Serk.
edit:
here's the last thread I recall about the "Serk cut"
http://www.valkyrieforum.com/bbs/index.php/topic,18294.0.html