My TTI's don't leak at the slip joint either. And there wasn't anything in the instructions about using any type of sealant. I did however coat the end of the slip tube with and industrial grade high temp anti seize lube and it appears to work well. They have been on the car for over 4 years and the tube came out of the collector pretty easy when I had them off this winter to replace the gaskets. I don't think I would want to seal it up with any silicone because one day they will need to come off and it will be a real pain to get loose!
