Not really, no. No aftermarket products have ever been implicated in serious child injury (in any lawsuit or study that I've seen). Except I think there was a 3 point infant seat with a bundle me that the child actually was ejected completely (it's a slide some techs got to see in their class, just the empty seat with the bundle me-type product, I don't know how badly the child was injured, though).
There's a good blog entry about seat protectors that DO interfere with good installations, though...
http://carseatblog.com/?s=protectors
Mostly I try to couch it in terms of common sense... a pad under the baby can interfere with the harness, thick strap covers prevent the harness from being as tight as it should, thick mats on seats can lead to a poor installation, mirrors and toys may break loose and hit a child or other passenger. Knowing that 99% of people are making some other mistake with their seat, a product like this might be the straw that breaks the camel's back, so to speak...maybe the loose harness wasn't bad, but adding JJCole strap pads makes it just that much looser and increases the risk of injury, for example.