It's true that the length of the shift lever below the fulcrum point determines the "throw" of the mechanism. As you point out, this length is the same for the sedan/P5 and the MP3/MSP shift levers. However, the length above the fulcrum point also has an effect since your hand on the shorter lever moves through a smaller arc when shifting.
I can confirm this. I just replaced my stock shift lever in my sedan with the MP3 lever. I also have the Kartboy shifter bushings. My shifter now feels quicker with less play in the mechanism. One final point is that I'm also using the MP3 shift knob which tends to be a bit tall. It looks to me that the Sparco knob used on the MSP would allow your hand to sit even lower on the lever effectively shortening the movement a bit more.
For me, this was a worthwhile mod that definitely makes the car more fun to drive.
02 DX Millenium Red
MP3 Strut Tower Bar kit
Kartboy Shifter Bushings
MP3 Shifter
Red Line MT-90 Gear Oil (stick now moves like an eel in oil)Suvlights Heavy Duty Wiring Harness
Osram Silverstar H4 Bulbs
5Zigen FNO1R-C 16" Wheels
Yokohama AVS ES 100 205/45-16's
Modified OEM Air Intake