well yeah theres more places it could be but u said it was aftermarket which means it came with new gaskets. and i just assumed that it was a bad alignment in the gaskets. it happened to me the first time. just make sure u torque things evenly. dont torque one bolt completley then the other, do it a lil bit at a time

and yeah at higher rpms ur goin to smell it more because u have more pressure in the exhaust so u end up pushing more gases through the leaking area