The nacelles offer space for the Warp Coils, which warp the time-space. You do not actually need these nacelles as you can integrate the coils in the main hull (see Defiant, Andorian Kumari class etc.)
Once there was that philosophy that an even number of nacelles is required, but that could actually mean an even number of Warp Coil lines. Thus the Kelvin would have 2 lines of Warp Coils, but simply integrated in one humongous nacelle.
The actual top speed and endurance would rely on the number of coils and their size relative to the mass of the starship plus of course the advancement of the Coils, as newer could mean smaller but just as efficient.