Nacelles are a form of power output. Much like wheels on a car. An extra nacelle does not produce more power itself, but may indicate that a ship has a more powerful warp core. Look at a dually pickup truck. Do the extra wheels give it more power? No, the truck uses its power from the bigger engine to turn the extra wheels for more traction. I think nacelles are like treds on tanks, they grip the fabric of space to pull the ship along, an extra one would give more strength to the pull. I am not sure, however how the efficiencies work. Probably for each added nacelle there is proportionally less added speed. That would be why most ships only have 2 nacelles, the efficiency of having the third is just not worth it, unless you are the Galaxy X or Stargazer. I think the formula would look something like this:
y = (log x) / x
Where "y" is some measure of "subspace traction or velocity," and "x" would be your number of nacelles. I am missing a horizontal stretching constant somewhere, but I think you get the picture.
The use of extra nacelles would be a way to even the playing field for heavier ships that can support a bigger warp core to reach nessesary warp velocities, otherwise, two nacelles is your best bet.
That is my take on it, from an engineer's standpoint anyway.