Isn't the nominal impedance that has to agree with the crossover. The crossover is interactive with the actual impedance (as in the varying with frequency thing) of the driver. So basically, more to it than an 8 ohm driver. A similar driver might work similarly, but the crossover might not even have been that suited to the one in there. Its another crapshoot like sticking a speaker in a box, there are simulators and such to figure, pretty much the same deal. on't actually know how much difference it makes in practice.
Edit: Actually, it is pretty similar to the box tuning thing, because its another resonant resistive/inductive network thing.