In my (small) experience of this, replacing plastic with bone or one of the better synthetics really opens up an acoustic. I first did this on a Sigma DR28 which was nice but a bit dark. I replaced both the saddle and nut as the nut had some tuning issues as well. The increase in volume and tonal detail was marked. I'd really recommend it but it depends on the guitar and bear in mind that the fit and finish of the saddle will probably have more impact than the material used. The nut material only affects the tone of the open strings so is more important if you live mostly in "country and western corner". I see no disadvantages to having a factory saddle replaced by a good repair guy by hand, the fit will always exceed the factory fitting.