I was wondering if anyone can tell me the benefits of putting the shock cords around the mid sections of my 300x's.

Keeps the clamp bushings from taking as much of a beating.

From the loading and unloading. In and out of the water.

I have them on my twin ob cat.

If I use 5/8 cord how many times to I wrap around them? I was thinking like 4 times?

I Would suggest 6 to 8 raps but unless you are running in rough water at a constant high speed ( like racing conditions ) then it would benefit you. If not I wouldn't worrie about it