Well, based on wanting
better wind penetration, more distance, and a better ability to cast weighted flies.
the z-axis seems like the way to go, but I don't know details about how it would do with light tippets. I would guess it woudl be fine with 5x, but 6x for example might be pushing it.
I don't think the ZXL will feel much like the bamboo, as it will be much faster and lighter. Then again, its not designed at all as a nymphing rod, and I'm fairly sure its only available in a 7'10" length.
For what you're after, a powerful longish 4wt that's not too stiff, I would also look at Sage's VT2 series, and Scott's E2. They might not be quite the trim level of the other's you're considering, but they sound like a better fit.
Come to think of it, our main sponsor could get you the 8'6" version (FR1024-4SD) of this rod G Loomis 4wt Stream Dance Metolius fly fishing gear
to try out. Also might be ideal.