I don't know how much heat the mirrors will handle. They are just plastic. They would work great as reflectors EXTERNAL to a batch water heater.
The one that I have sitting in my garage (which is about 20 degrees farenheight right now...) has warped a bit. I think the plastic backing and the mirrored surface expand and contract at different temperatures, bending the mirror.
So, if there was an application where the mirrors themselves get hot, the big problem might be keeping them flat and properly mounted.
I saw a great, simple solar technology at:
IWillTry.org Build a heliostat for solar heating and lighting
It's basically a big reflector that tracks the sun and constantly blasts a beam of light through the north window on the guy's house.
I have poor solar access at my house, but I LOVE this concept!