I first decide what is the layout for. That was for PHP, HTML and JS. So, I create a program to calculate the amount of each character that exist in HTML, PHP and JS source code and the most popular characters have to be easily typed. I did pretty much testing and I decided some of the characters entirely based on my instincts (like the \ ; ' and $). Then I just install the keyboard layout and switch it to PHP Coding Layout whenever I am about to start a coding session.
There's some logic, like the (), {} and [] are near each other but more interestingly look at the < / - and > characters, the reason for their position is that I wanted to keep < and > near each other, but since I code more PHP I decided to keep - and > next to each other so you can type -> in PHP code easily, but still maintaining an easy way to type tags in HTML <tag></tag>.
I have come to the point that there's no ultimate layout, it's a lot about your personal taste and the way you type. At least I don't type the way other people type, I use my fingers differently and I use only 9 fingers. So, I guess it depends on how much your hand moves while you type, how much you use each of your fingers, etc, but you can certainly boost development time if you learn to type some layout that is designed for coding, unlikely qwerty, which was designed to be as slow to type as possible so that those old type writers did not mash.