I would love to see/shoot a X-Pan and a full frame camera side by side with the appropriate lenses to what, if any, differences there are. This may not be backed up with measurements and calculations, but I often suspect I'm getting a 'fake' x-pan mode by cropping to 65:24. Otherwise, why even bother with a X-Pan that has double width frames, why not just shoot wider lenses and crop?
It's similar to a 17mm lens on micro four thirds, it's supposedly a 35mm equivalent of full frame in terms of field of view, but I wonder how much the lens design accounts for the look of the image.