There is also the fact that the speed of light is an intrinsic part of the relationship between mass and energy made famous by Einstein's equation E=mc^2. If the speed of light was that much greater in the past then so too was the amount of energy produced per unit mass in the center of stars. This has very obvious consequences. If c-decay is true then we should see stars that deviate greatly from what physics predicts, but we don't. Not only that, but this also indicates that the decay of isotopes on Earth in the past was much, much more energetic leading to massive increases in both radiation damage and the overall temperature of Earth.
Needless to say, creationists have not thought this one through very well. The consequences of an increase in c is the utter destruction of all life on Earth, not to mention stars across the galaxy.