when I get this game I will be playing it without the Zapper shell.
earlier today I wanted to really see if it would help, so I got my old Wiimote desert eagle shell (its that white one that looks like a normal handgun) and put in Wii Play. I tried several times with the gun shell going from one hand to two hands on it to see if it would help, and after a few minutes with it, I got an alright score, nothing special. Then I tried it again with just the wiimote and no gunshell and first attempt I went past even double my score with the shell.
I actually do play a lot of light gun games on a variety of systems since the NES, but I just dont think a shell for the wiimote is a better solution for aiming and all it will add is the idea that you are holding a gun like toy.