I have owned both the PX and the PX7 (and also the P7, P7w and P5s2).
I would not say that the soundstage on the PX8 is smaller in any way. It is not small for a closed back headphone, not at all, but sure there are wider sounding headphones (Focal Bathys being one of few that can actually match the PX8 in other areas).
Perhaps the PX was a little wider in this regard, but the PX are inferior in just about every other dimension.
However, the B&W house sound and tuning is by most people perceived as more forward, more like "speakers in front of you" rather than a circumaural and multidimensional surround sound that some headphone manufacturers strive for. Personally I like this for acoustic music, rock, female vocals and other genres that benefit from the excellent mid range of the PX8, but I know some people prefer a more expansive sound, e.g. for electronic music. For what I listen to this mostly sound synthetic.
I would guess that if you really like the PX you might feel overwhelmed by the presentation that the PX8 have (more of everything) anad if you truly enjoyed the PX7 you will be more than happy with the PX8 which is a far superior headphone in just about all aspects.