First rule - do not rely on salesmen for fully accurate information.
The first thing you do need to consider is what is that item's STC rating.
- The STC rating is equivalent to how much it will reduce common noises.
- So a 30 is equivalent to 30 DB of noise reduction while a 60 doubles that.
Now as for the marketing pieces - they do have a slight point on some frequencies that are not reduced or represented in that rating. With that said it depends on what the issue is, the source is, etc.
In most cases cork is more than adequate and actually adds a little insulative value.