The silicone gasket will scratch of easily with your finger nails. You should have a copper or aluminum gasket there. Until you get a good gasket and make sure your exhaust flange is straight you'll get no dice.
A copper or aluminum gasket will expand correctly with the heat of the exhaust and stay sealed. Better than a paper gasket.
I use the Permatex® Ultra
Temp RTV Silicone
Item # 81878
covers, oil pans,
The exhaust is tough to seal with out the right gasket. The cheap china mufflers take nothing to warp the flange. Put a strait edge on it and check.
This product works perfect only when each surface is flush [not warped]. Other wise a gasket has to be used.
It takes very little of this product to do the job.
I have a carb manifold with no gasket just the said stuff never leak in 2000 miles. Something always rattles loose on a china. Having a small tube of this for a needed field repair can be nice. [when the cheezy china gaskets fall apart.]
All this being said Make better gaskets than the stock gaskets and that's a great approach.