Taking the diameter of the Steeda UDP crank pulley (4.875") as a base number, using only the crank pulley would result in under-driving both the water pump and generator by 25%; vs. the -39% (w/p) and -32% (gen) reductions achieved using the complete UDP set. This is not an insignificant reduction so some effect from using just the UDP crank pulley will be had.
That said, UDPs do not add any horsepower they simply reduce the horsepower required to power the water pump and generator--by reducing the speed at which they run--freeing up that HP (about 1.5 to 2.5 HP max, normally 0.25 to 0.5 HP) to push the car forward. This very small gain is noticeable only on hard acceleration and higher engine speeds, and even then not all that noticeable.
If you are the sort that never "floors it" and always shifts at 3k rpm then don't bother, you will never know they are installed. Fuel economy at cruising speeds will improve a tiny bit, maybe 0.25 MPG...