Ask him why ?
I dont have an exhaust leak and get popping this never happened with stock exhaust and cat actually I think the bike was probably just as fast with all standard
I only replaced exhaust because it was starting to rust and cat box was cracked I dont think I would change anything if I had a new bike again unless of course it was to put a silverback kit in.
I think I would buy some new gaskets and some correct size stainless toggle clamps and have another go the exhaust paste is a good
short term fix but job is best done right
also being Scottish the fosters can is probably cheaper and more readily available than gaskets so I might try that first good hardware store will have toggle clamps