That's an amazingly subjective question. I'm assuming there's no Apple Store near you to try it out, but lets just say that I was willing to punish myself with the experience of running FCP X on the iMac in my signature. You should be able to get an idea of the performance difference from there.
FCP X on the new 13" Haswell, especially with maxed out ram, would be "fine," but rendering and massive projects won't run like greased lighting. See Mac Pro for that.
The ram will help with the editing experience, while processing power would speed up rendering time.
For me, I did everything I could to optimize the editing workflow, and let rendering take its sweet time.