Howdy.
I figured I'd respond since the last response didn't help very much. 🙂
I used Logic Pro 9 on a 2011 11" i5 Air for the last 2 years.
It worked great and I really only thought I could use more power when I had multiple instances of say Amp Designer running. Even then, it didn't max it out, it just started to creep up kind of high on the CPU meter.
I mixed pretty large (over 40 tracks) projects on that 11" with no problems.
The current i5's are a bit faster than that 1.6ghz i5 I had in that machine.
I just picked up my new 13" Macbook Air yesterday, and I went with the i7. HOWEVER, that's also because I'm entering a graduate program in music technology in the fall. I would say unless you plan to be super ambitious with massive projects and lots of CPU hungry plugins, then the i5 (with 8GB ram) will be fine.