If you want massively in-depth reviews of hardware, with detail a few steps beyond the "OK, this is almost too much information," AnandTech has been the place to go since they launched 16 years ago. Their iPad Air review continues this tradition, and while the review is basically in line with the vibe of all the other reviews on the internet ("The iPad Air is great!") what is of particular interest to us is the amount of effort Anand went into with testing the GPU of the iPad Air.
AnandTech notes that the A7 in the iPad Air is clocked higher in the iPhone 5s, with many of the benchmarks show the iPad Air taking a small lead over the iPhone 5s and a 40% to 70% improvement over the iPad 4. Interestingly, when it comes to rendering triangles on screen, the iPad 4 actually pulls ahead. Anand suspects this is because of differences in how the A7 and A6X handle their rendering pipelines. That might seem bad, but the author mentions that they "haven't seen any real world cases where it matters yet."