Truthfully, this will do nothing in the area of stiffening. xB's have shocks in the rear, so a "strut bar" is more of a showpiece than a functioning item, as it connects to the studs on the shock (which I was never comfortable with). I have a set of home-made trunk bars that offer a degree of body
stiffening, but if you want chassis stiffening, go with your latter idea and get the sway bar.
GT Spec used to make a rear "sway bar for the xB.
And years ago, an engineering buddy of mine who was a member here made one (which he gave me when he sold his box - it's still
sitting in my garage).
For the record, my home-made trunk bars.