OK, I'll bite to get a post on this age old question.
I've tried Bone, Tusq, & Ivory on any number of Taylors I've owned. I've not kept an exact count, but about 50% of the time I've gone back to the factory Tusq. The only way to tell IMO is to try it out and see what happens. The good news is that if you do the installation yourself you will have a minimum investment in the new saddle (Bob Colosi is a good source) and can easily change back if you so desire.
My All-Mahogany GC BTO retains the Tusq, while my '94 612c sports a Fossilized Walrus. I've kept a stash of the various saddles I've tried and rejected; so now I can try them out at will.
BTW, I've never had the nut replaced, but I suspect the change in tone would be minimal when compared to a new saddle and the cost would be much more.
Give 'em a try, not much to lose.
Gary