I would say not worth it. Very minimal ghz boost you probably won't notice and an additional 2MB (6->8mb) of L3 processor cache you probably won't notice.

I haven't seen any benchmarks, but I doubt you'll get a big return going to the i7, certainly nowhere near the return on investment the dual core i3 to quad core i5 is. Macbook air is mostly going to thermally limited under sustained load anyway, so the i7 benefits are likely pretty limited.