Ok, so I'm late to the party, but I read an article from a guy in Houston, TX that may interest you.
Cool Roof Design for Hot Texas Climate | Houston Cool Metal Roofs
You might want to give this a few minutes of your time. I found it very interesting and will try to do this if/when I replace the galvalume roof on my 12 year old house.
I am a huge fan of foam insulation, both my home and my garage / workshop project were foamed. I just did the foam the lazy way. Traditional framing and foam the entire inside envelope. Sealed up nicely and did a pretty good job on a blower door test. My remaining air exchanges were from a leaky French Door install in the MBR, the vent for the gas HWH and the through the floor drain for said HWH. Still, a very energy efficient home and it's very comfortable too.