Hi.
First post here for me, but since you haven't had many replies...
"Yes", I have a one-week old loaded MBA 11inch(!) and have been running fcp x on it for the past few days.
While i can't compare the speed to other macs, and I'm no editing professional, it seems to run very well. Certainly a lot better than the Vegas setup on a quad processor windows desktop i had been using.
I have a wee 4 minute clip with 68 'segments' of 1080p film (the largest being 250mb). Coupe of layers of effects, sound etc. I use a 27 inch non-thunderbolt display most of the time and the only complaint is that the fan runs non-stop once things heat up.
Editing is fast, rendering 1080p takes about 4 min. I'm impressed. Still figuring out how to do a few things like masks in FCP.
I use my MBA as my personal laptop and for Photoshop. Came from a 2010 13 inch loaded MBA whose performance blew me away with PS CS5 (many, many 21mp photos with multiple layers). Decided to go with the 11 for portability this time since the specs are similar except for battery life.
DB