As a MacBook Air and Pro Retina user, I can say that for 1080p30 footage from a GoPro camera; editing is just fine with FCP X on an 11" MacBook Air provided that you get the top end processor and memory combination.
For the 11" Air, that is currently the i7 2.0 (boost to 3.2)GHz processor with 8GB of DDR3L RAM.
Also, upgrade the storage. 256 SSD is minimum for editing substantial projects locally, if you move the project to external storage once done with it to make room for the next one.
And on a maxxed out Retina Pro, editing is *fantastic*
Even on a "base" model 13" Retina Pro, editing is still super fast. The display on the 13" and 15" Retina models is the best I've seen on any laptop EVER.
I'd spring for the SSD... Mainly for speed, but durability is greatly increased too!
Yes, I know, money, Apple tax, whatever.
I've made the mistake of trying alternatives several times and they do save me money until I realize they suck, which has been every time.
YMMV