The iPhone 4S was unveiled in 2011, and ever since, Apple has included an iteration of that 8 megapixel sensor in each consecutive model of their phone. The iPhone 6's camera equals or outdoes its seemingly superiorly specced rival phones, basically because Apple has heavily refined their camera (making pixels larger, increasing the aperture, more light etc etc). Basically, its an exceptional camera for a phone, and judging it on it megapixels alone is wrong.
That said, perhaps due to Apple's love-affair with thinness, you lack optical zoom. Instead the iPhone 6 sports digital zoom, which deteriorates the quality of your image the more you enlarge. So that's a massive negative. Also, the price of an unlocked iPhone with decent storage is incredibly high (for poor Australian me, the 64GB iPhone 6 hovers around AU$999), so unless you want to be roped into a plan, you might as well seriously look at some DSLRs.
That app you found says in its description that it has:
Full Camera Control
• Full 1080p and 720p support on selected devices
• Front and back camera support with full focus control
• Camera zoom
• Automatic or manual white balance and exposure control
• Cinema Mode (2.35:1)
Essentially everything you need for brickfilming. But frankly, I say just test out your iPhone 5 with the app, and if you are getting a 6, it should work too, but you'll be making some sacrifices (e.g. zoom). I will stress this though, don't just go out and get an iPhone 6 specifically for brickfilming because you definitely can get a very good DSLR for a lower price.