The speed of light in a vaccuum I always think of as the tension of spacetime - like plucking a guitar string - hard or soft doesn't matter - it will vibrate at only one speed for a set tension.
The speed of light can be directly caculated from Maxwell's equation (its a page of tricky maths)
http://www.phys.unsw.edu.au/einstein...e3_Maxwell.htm
But that is a very powerful proof when that field property can be directly determined by the key forces shaping the field itself.
PS
Apologies above - mis-spoke its Homogenity, not Isotrophic behaviour that science is questions - still around we could be in the middle of one or more "voids" (low density areas) around 300 million light years to 2 billion light years diammeter. Tests based on COBE in the next decade with more sensitive space telescopes might rule this model in or out.