HIGHLY doubt Subaru would offer a stripped down model in America, but if it was sold I'm sure someone would buy it. It's not a very american market thing to do to offer stripped down models. Only hardcore track enthusiasts would want them. And there aren't many of those.
The main motive would be cost savings. A stripped down model would probably cost less.
Weight savings you can do to your own car. It would just be a waste of money to throw out brand new parts on a new BRZ.
Who is really going to switch the bumper out? There aren't any aftermarket bumpers out yet. And most aftermarket bumpers are fiberglass. A proper diffuser bolted onto the stock bumper will do much more then any aftermarket rice bumper.