I have the exact set up, P5 and 700 arm. I have the full Groovetracer product line and am very happy with it. Of the subplatter, platter, and counterweight, I noticed the least amount of improvement with the counterweight. However, low level details especially in the bass was observable with the Groovetracer counterweight.
I'd say get the Reference subplatter if you're going to get anything - it made the most obvious improvements in cleaning up and stabilizing the sound, decreasing wow and flutter, and decreasing friction during startup of the motor. The acrylic platter also made a significant improvement, and really brought the performance up a couple of notches, showing how colored the stock platter and mat are. The counterweight finally does add some further refinement, but perhaps not as much as the other pieces.
I'd say get the Reference subplatter if you're going to get anything - it made the most obvious improvements in cleaning up and stabilizing the sound, decreasing wow and flutter, and decreasing friction during startup of the motor. The acrylic platter also made a significant improvement, and really brought the performance up a couple of notches, showing how colored the stock platter and mat are. The counterweight finally does add some further refinement, but perhaps not as much as the other pieces.