how do the new B&W speakers stack-up against the Alpine speakers from the 2007-8 model XK?

Would it be worth it to swap them out?

 

Most likely, a good set of B&W will out-perform a set of Alpine.
However, speakers fall into the extremely subjective opinion category.

I personally love the sound of B&W and have 801S2 Matrix front with full Matrix surround of approximate same era for a 7.1 surround system. When I had a house where I could have outdoor speakers - B&W again (sadly sitting in a box in a closet right now).

 

I just upgraded to an 08 Portfolio with B&W and it sounds amazing compared to the standard audio my 07 XK had. But I don't think that's just from the speakers. The amp has dolby 7.1 so I think there are a couple of extra speakers (center and sub) and there are a bunch of extra controls in the touchscreen. If you have the volume a bit high the door skins don't vibrate and rattle since the bass is coming from the sub and not the door speakers.