Not sure about diminished value but I found some interesting things about Carfax with my last car.
I had a couple oil changes then at 24,000 miles, complete engine failure (VERY common for the 2.0T in the Cadillacs).
On the Carfax, the oil changes all were reported as "Vehicle Serviced". #4 piston grenaded so I needed a new engine, head and turbo. Carfax reported...you guessed it "Vehicle Serviced". There was literally no distinction between an oil change and a blown engine. I got an amazing trade-in value for it anyway so I take Carfax with a grain of salt now.
I just got my SS back a few weeks ago after $9100 damage from a deer hit. This happened 3 weeks after taking delivery and I haven't thought twice about diminished value. The car looks perfect now and the last thing I'm gonna worry about is a lower resale value.