I'm curious about this as well. I'm going to start iOS development after I finish software engineering this year, and I'm curious as to whether the i7 will have any benefit over the i5, even just the 2.9GHz configuration.
EDIT: I mis-read your post; you're looking at the 13" model. So disregard the comments about the quad-core CPU (although if you can swing it, the 15" model is going to be much faster). The rest is all true though. The faster i7 is going to accomplish tasks faster. But the base i5 will still accomplish tasks.
The two extra cores is the big deal here. A quad-core CPU is going to perform a lot of tasks quicker; tasks like compiling your applications. It's up to you whether that's actually worth the money or not. There was a time when getting the slower CPU meant there were applications you simply could not run. Today? Not so much. Even the base i5 in the non-touchbar 13" MacBook Pro; the slowest of this line, will essentially run every application that WILL run on macOS Sierra. Some won't run smoothly, but it'll run them all. And certainly all of the i5's on the 15" model are more than capable of even the most intense applications. The big difference is going to be time to complete intense tasks, like rendering and compiling.
I do some light duty video editing for my own YouTube channel, NOTHING serious or even remotely "pro". While I did decide to invest in the faster quad-core 15" and the dedicated GPU in part to improve performance there; I didn't actually
need to. I only ever do one video at a time, so leaving it for a few hours (which is what it could take sometimes on my 2012 MacBook Pro, dual core core i5) was not the end of the world. But if I were producing several videos a day, that would be unacceptable. Of course, FCPX can add tasks to the list and I can keep working while it background renders, with a dual-core 2012 MacBook Pro I could conceivably backlog it so much that I'd have to leave it running overnight to finish my work.
I don't do any iOS development; I used to do some coding (just as a hobby) years ago; and things have surely changed. But I would assume for bigger and more advanced applications, you're going to shave a lot of time running the faster CPU. The 2.9GHz chip also has a lot more cache which probably won't make a huge difference in Xcode, but in other applications, that's a difference you can "feel". Applications have more cache to load up which speeds things up. There's a few places data an application needs can reside. The hard-drive (SSD), which is the slowest and farthest away. The RAM, which is significantly faster, and the cache; which is the fastest and is one clock cycle away from being executed, it's instantly available. Apple used to make a HUGE deal about the cache in PowerPC chips and how much faster that made Mac versions of applications vs. the Windows versions (they used to do side-by-side demos of the fastest Windows PC's money could buy, often twice the price of a Mac, running something like Photoshop, and the same app running on a mid-grade iMac or PowerMac with a PowerPC CPU; and the PowerPC machine would get the job done a lot faster. And then usually a maxed-out top of the line PPC Mac just for the wow factor).
So tl;dr- the slower CPU and the faster CPU will do the same work and run the same applications. You are not going to run into a situation where "I can't run this application / use this software / perform this task because my CPU isn't fast enough" on any 2016 MacBook Pro. However, the faster CPU will accomplish the same tasks in a lot less time.