The Hevi13 will absolutely kill but they don't pack the same punch as the Fed heavyweights. You'll get more pellets will the hevi13 though. I use the Hevi13 #7 in my 12 gauge and it kills them out to 40yds. Those are the exact same pellets coming out of the 20ga but less of them. If you can pick up one box of the Hevi13 and pattern I would recommend that. If you get a killing pattern, use them.
With my gun, I got better patterns with my RSF than the SSX. Yours could be completely different though.