i don't use an xpan, but 45mm on panorama mode is the equal of a pretty wide angle lens and with all very wide angle lenses you are going to get some vignetting -- it is the physics of the beast, light falls off the farther it has to travel (something about inverse proportion to the square of the distance, or something like that) and when you think about how far off to the side the center of the lens a wide angle lens has to throw the light, it gets dimmer out there, hence special filters to make the center darker so it all evens out.

Early super-wide lenses for Leica and Contax had the same problem. The current 21mm lenses for M Leicas still do.