I use a 2012 Macbook Air 13" with Logic Pro X without any problems (1.8, 4GB, 128GB SSD). However my usage is pretty basic: 10-20 tracks of audio with 2-3 plugins each and no synths or virtual instruments. I use an external display when in my studio.
I recorded my whole album on this machine using a Tascam US-1800 via USB.
Compared to my father-in-laws iMac (3.4, 8GB, 256GB SSD) that I occasionally use, the Macbook Air is slower regarding project loading/saving/bouncing. But on both machines, Logic Pro runs exactly the same. No drop outs, no clipping or glitches.
I can say for 100% certainty that my usage of Logic on the Macbook Air does work without any problems, performance wise. I spent two weeks on a cruise ship and used my Macbook to mix and arrange some tracks while on battery power, using headphones, without any problems whatsoever.
That being said, if you are seriously going to use Logic for professional production, then in my opinion a Macbook Pro would be better for the following reasons:
Better display
More RAM (16GB)
Faster CPU
Since, in the real production world, time is money, and a faster computer is money well spent. You sacrifice portability for power with the Macbook Pro, but if you are making money on your projects, then portability is less of a concern. Honestly if you were going to be professionally producing music I would use your iMac instead. If you are going to run many virtual instruments or synths, then the Macbook Air would possibly not be powerful enough.
However in my case, I use the Macbook Air for all my tasks (office/work/school/music) and I have not had a situation where I was thinking "geez this computer is slowing me down".
Regarding the i5 vs i7.. Thousands of opinions exist on this debate. In my opinion, the real world difference is not worth the $. You said that you have external hard drives for more space. I would spend the $200 i7 upgrade money on a larger internal SSD. I can tell from personal experience that my 128GB SSD is way to small. Some might argue that the i7 would be better for Logic regarding the usage of plugins, synths, etc. My counter is that a Macbook Pro would be better, not an i7 Macbook Air. I would not buy an i7 Macbook Air if you need more power; buy a Macbook Pro, or iMac, or MacPro. The real world difference between the i5 and i7 on a Macbook Air is not worth the money. Save that $200-300 and buy the Macbook Pro with it.
Again, small projects like mine (1-5GB in size) load reasonably fast (5-20 seconds) and save just as fast. If you have many tracks, VIs or synths or plugins, then you are justified buying a much faster machine like the Macbook Pro. For me, I wanted lightweight and portability.
If i were to "do it over again" I would STILL buy the Macbook Air, except with 8GB RAM and a larger SSD. And I only want more RAM because I use VirtualBox for running Windows and Linux in a VM. But I made a mistake by not spending extra money on a larger SSD. That would be the only thing I would change. Using external HDD works, but it is definitely slower, and that is something you will notice. My next laptop will be a Macbook Air (or the equivalent) with as much on board storage as possible. The baseline CPU and RAM will be "good enough" for my music projects.
Good luck in your musical endeavors. Just remember that its the music that counts, not on what kind of machine you record it on.
