Nope, Rapid Fire macros will actually slow down fire rate on a fully automatic gun. This is good especially for conserving ammo and you always need to conserve in zombies.
Rapid fire for fully automatic guns are a Plus, it gives you way better accuracy. It wont make the bullets spread as much, guess you can say it keeps them more in a bunch then scatter all over the place.
Get a fully Auto gun fire at the wall then switch it to rapid and fire it next to the same spot when you fire it with out rapid, Fire. You will see a big big difference.
When i had a ps3 rapid fore i use to use my rapid fire on the mp5 in COD4 and it was deadly.