The XRS is based on a X10 chassis whereas the X11RS is based on the X11.
The x10 doesn't use the alloy hinge pin braces and the change to these would means new chassis, alloy blocks, wishbones etc.. So to upgrade the Xrs to the x11 spec would be very expensive and not really worth it. But, there are a few options that you can upgrade to.. The X11 alloy cross brace, big bore shocks, x11 wing mounts and I'm sure a few others.
Saying all this, there is nothing wrong with the X10 so you'd have a good car. The only thing I would say worth thinking about would be the cross brace upgrade kit as this strengthens up the front pretty well.
http://www.the-pred.co.uk/zencart/in...products_id=85