This why I sell the corks I do. Not all corks are alike. I have sold over 250,000 of my corks and I have only had one occassion where they leaked. In this case, the wine maker soaked the corks overnight and used a fishing line to make sure all of the gas escaped. The leaking was due to the fishing line leaving a ridge in the cork. The ridge was caused due to the corks being soft from the overnight soaking. As a result, I still claim that my corks do not leak.
I might also add that I have several customers that all they buy from me aremy corks. They buy their other supplies from my competition, but I am their source for corks.
The one complaint I have had is that they can be difficult to remove. I have asked my supplier to increase the coating which should help. On the other hand, I have learned a little trick. Use a knife to score around the top of the cork where the cork and bottle meet. For some reason this breaks the seal and allows the cork to be removed much easier.