How to avoid sluggish UI in FCPX - more CPU, GPU, RAM or what?
Your hardware is NOT underpowered. I can run FCPX on a old MacBook Air using proxy media, with only 4GB if memory. That machine is much slower than yours. So it's not the MacBook that's the problem.

Have you tried to copy your media to a local drive just to see if you have issues with your raid system?

Another thing to check is running the Activity Monitor utility and see if your CPU is busy when you get the beach ball. 

Before we know why you are having these issues it's hard to give a recommendation on what parts to upgrade.