I agree with many of the comments, for $100 bucks a 14% boost in CPU power seems like a pretty good deal. The thing to think about is what your using your MBA for?
Reason I say this is, if your not going to use your MBA for CPU intensive tasks such as video encoding, photoshop processing, number crunching etc... you will never see the benefit of the 14% speed. Unless both cores are running at MAX you won't really notice a difference. Daily tasks such as web, youtube, playing a movie from the drive or streaming from Hulu, Netflix etc... They simply don't even tax the CPU to 100%. They rely heavily on the GPU, which is a decent one. This is no different that those people that buy a quad core iMac and then do nothing more than web, email, FB and a few games. Rarely if ever even using the full power or all the cores of the CPU. The ram and solid state storage will be the biggest noticeable factors that let you "feel" the speed in the UI, switching tasks, having multiple programs running etc...
The first upgrade I would do is ram, bar none will be the best 100 bucks spent on the 11" MBA, CPU is one of those, if you can afford it why not. Either way it's 1.4 or 1.6Ghz, it's not breaking any speed records decoding or converting something in Handbrake lol. It is what is it.