Would it improve the performance SIGNIFICANTLY to remove the crossover and rewire the system using good quality speaker wire and new binding posts and then bi-amping with an external active crossover?Yes it would, although "significantly" is a relative term, isn't it!
However, talking about it is easy. Implementing it is not. You might try improving the matls used in the passive xover first.
Anyway, you need:
the original x-over schematic,
the tech sheet,
info on the drivers and, not least,
help from an experienced spekaer diyer or speaker tech. Enjoy