Depends on how you develop. There are many ways to improve productivity that do not require faster machines. I'm a lisper for most work so I rarely have to wait for any compilation. Every interaction developing with this computer is basically instantaneous.

Also, I find that new developers think they would benefit greatly using the fastest machines to learn to program, but then never develop a good sense of algorithmic complexity as everything just screams. On slower machines you can have the opportunity to feel the difference between O(1), O(log n), O(n), O(n^2), etc.