Nepheline Syenite has been a standard in the ceramic industry for many years, and is very popular for its whiteness. Nepheline syenite melts lower than feldspars. For example, it is possible to make a very white vitreous medium temperature porcelain (firing as low as cone 4, but more practically at cone 6). Up to 50% nepheline syenite will be needed at cone 4, 35-40% at cone 6. 20% silica is needed or glazes will craze. The rest is clay, preferably kaolin. The whiter the kaolin, the less plastic it will be, and the more there will be a need to add a plasticizer. White plasticizers are expensive (VeeGum T at 3-5% will be more expensive than all the other ingredients combined). Bentonite plasticizers are cheaper, but will darken the colour. You might consider using a ball clay instead of a kaolin, it will reduce the need for plasticizer additions, but the body will not fire as white as with kaolin.
Since nepheline syenite can be slightly soluble, in pugged bodies it can be responsible for stiffness changes during aging (although admittedly many other factors can also contribute to this). It can more challenging to maintain stable deflocculated slurry bodies using nepheline syenite than with feldspars. However, the place where you may note the solubility of nepheline the most is in glaze slurries containing significant percentages, they can gel over time and the addition of more water to thin the slurry can wreak havoc with application performance (try adding a few drops of deflocculant instead).
Because of its sodium content, high nepheline syenite glazes tend to craze (because of the high thermal expansion of Na2O). Also, since nepheline syenite has more alumina than most feldspars, substituting it into recipes means that on one hand a lower melting temperature is achieved while on the other a more viscous melt results because of the extra alumina.
Body Maturity - Body Flux
This material is generally fluxes better than feldspars and produces whiter burning bodies.