We keep talking in circles. I finally decided to waste 5 seconds looking for benchmarks for an i7 MBA vs. an i5, and found this (from 2013, but the general relations between the i7 & i5 should still hold).
http://www.anandtech.com/print/7113/2013-macbook-air-core-i5-4250u-vs-core-i7-4650u
In particular,
Bootup time: yes, the i7 benchmarked faster than the i5 -> 11.4 seconds for the i7 vs. 11.7 seconds for i5. But in everyday life, is this really a noticeable difference?
The benchmarks also show that the i7 is faster at doing more intensive CPU tasks, BUT...at the expense of battery life.
Before people go "But the i7 will get it done faster" the author note (regarding the battery life benchmarks):
"What's important to note about all of these tests is that the amount of work done per cycle of the test doesn't vary based on performance. There's enough idle time baked in to make sure that the Core i7 based 13-inch MBA isn't artifically penalized by having to do more work than the i5 model simply because it's faster."
He also noted earlier (this is just above the nice chart that shows the i7 runs at higher frequencies AND higher voltages, which as he notes below, is the worst possible combination):
"To understand the impact on thermals (and battery life) of the Core i7-4650U on the 13-inch MacBook Air you need to understand what's going on under the hood. To hit higher frequencies, the i7-4650U generally requires a higher voltage. Power consumption (and thus thermal dissipation) can scale linearly with frequency, but it scales quadratically with voltage. The combination of the two is quite possibly the worst case scenario from a power consumption standpoint."
So, with that in mind, the i7 actually beat the i5 on battery life for light workload tasks: 11.25 hours for the i7, vs. 11.03 for the i5. (note: this is about 13 minutes, or just under 2% difference)
However for medium workload tasks, the i7 was lower than the i5: 7.80 for the i7, vs. 8.93 for the i5. (14% difference in favor of the i5)
And again, the i7 was worse for high workload tasks: 4.68 for the i7, and 5.53 for the i5. (18% difference in favor of the i5)
I haven't seen anything more recent than this, but again, the general principles for i7 vs. i5 should still hold true.
So, reiterating awair above:
#1: SSD
#2: RAM
#1.5: buy a back up drive (my little soapbox)
#1: CPU