Also, what sonic complications are caused by say mixing a 25uf with a 15uf to make up 40uf since many manufacturers don't make 40's in certain compositions?None whatsoever, especially in a generic/general sense. Of course, I'm sure that out of the hundreds of thousands of capacitor types available, you could find some combinations that are problematic . . . but there's no way to predict this without knowing all of the very specific, and generally unknown/unpublished, electrical characteristics of each. But with high-quality film caps, you're on very safe ground.
Now on the other hand, changes to the crossover design itself have a huge impact, and if this is a custom speaker system, then that's a better tree to be barking up . . .