I was planning on bashing an -8P with one of the lance pack plastics, and this new 8X is noticeably chunkier that that old model. Not that it looks any bigger, just thicker in parts. It might look a little weird to just slap a head on top of it. Plus then you'd need to sculpt over or cut down the torso cockpit.
On the flip side, I'm now wondering how I want to bulk up the plastic model I have, seeing the proportions of the new metal fig.