Just thinking out of the box a little, since you are rebuilding them anyway would there be a slightly different wheel configuration that would fit in there that would be "a better match for our Z's"?
Clipping wheels?
Different wheel trim dimensions?
Housing modifications?
Is the GT2554R a pretty good match for Z's in its current form? I am not good at interpreting those graphs.
Maybe some of those Honda gurus have some tips on improving the GT2554R's design?