head.js and responsive web design

Question

I've been researching responsive web design technologies and head.js and I have a few questions.

• How much of a realistic reduce in load time head.js have you experienced when lazy loading external js files?
• How much extra development/effort is required to prevent Flash of Unstyled Content problems?
• Are all the CSS features of head.js useful in creating a responsive design (for different size screens, devices, etc)?
• Does head.js play into lazy loading of images and plugins?

Would people recommend something different when we are trying to accomplish the following things:

• Fast, efficient load times for a lot of javascript files
• Reducing development time and increasing UX for multiple devices, screen sizes
• Extensibility of a large scale application
• Following best practices

Any input is appreciated, thanks.