I also think you need to redo your math on the price per gflop for a Mac pro, ypou seem to be at least half the price of my back of the envelope work. Unless you have some crazy good supplier.
Exposing more detail behind this math is unfortunately not something that I'm ready to do, but I'm pretty comfortable with it in broad strokes. EC2 really is that much more expensive, when you factor in things like network bandwidth.
As I noted elsewhere, I mention EC2 because all of our (funded) competitors run there. We can split hairs over whether I could save 10% on Linux systems vs Mac systems, but the elephant in the room are all of the companies trying to make this sort of service work in EC2. You can't do it, and make money at the same time. Even if you can make money at small scale, you will eventually be crushed by your own success.
My overriding goal for imgix's datacenter strategy (and elsewhere in the company) is to build for success. To do that, we have to get the economies of scale right. I believe we have done so.
I also think you need to redo your math on the price per gflop for a Mac pro, ypou seem to be at least half the price of my back of the envelope work. Unless you have some crazy good supplier.