And since the performance plateau was reached @ -j7 in the test above, I tried with some heavier loads, too:

-8:

-j1: Average time = 23.259 seconds (3 rounds), Encoding speed = 464.85x

-j2: Average time = 12.256 seconds (3 rounds), Encoding speed = 882.18x

-j3: Average time = 8.570 seconds (3 rounds), Encoding speed = 1261.61x

-j4: Average time = 6.566 seconds (3 rounds), Encoding speed = 1646.66x

-j5: Average time = 5.374 seconds (3 rounds), Encoding speed = 2011.78x

-j6: Average time = 4.679 seconds (3 rounds), Encoding speed = 2310.59x

-j7: Average time = 4.207 seconds (3 rounds), Encoding speed = 2569.80x

-j8: Average time = 3.908 seconds (3 rounds), Encoding speed = 2766.87x

-j10: Average time = 3.732 seconds (3 rounds), Encoding speed = 2896.85x

-j12: Average time = 3.672 seconds (3 rounds), Encoding speed = 2944.44x

-j14: Average time = 3.657 seconds (3 rounds), Encoding speed = 2956.79x

-j16: Average time = 3.706 seconds (3 rounds), Encoding speed = 2917.17x

Performance peak here is -j10 .. -j12, so even when the CPU ran out of performance cores there is some benefit.

-8p:

-j1: Average time = 74.252 seconds (3 rounds), Encoding speed = 145.61x

-j2: Average time = 37.895 seconds (3 rounds), Encoding speed = 285.32x

-j3: Average time = 26.564 seconds (3 rounds), Encoding speed = 407.02x

-j4: Average time = 20.738 seconds (3 rounds), Encoding speed = 521.36x

-j5: Average time = 17.305 seconds (3 rounds), Encoding speed = 624.79x

-j6: Average time = 15.060 seconds (3 rounds), Encoding speed = 717.94x

-j7: Average time = 13.406 seconds (3 rounds), Encoding speed = 806.52x

-j8: Average time = 12.321 seconds (3 rounds), Encoding speed = 877.50x

-j10: Average time = 12.201 seconds (3 rounds), Encoding speed = 886.13x

-j12: Average time = 11.442 seconds (3 rounds), Encoding speed = 944.94x

-j14: Average time = 10.573 seconds (3 rounds), Encoding speed = 1022.64x

-j16: Average time = 9.777 seconds (3 rounds), Encoding speed = 1105.86x

-j18: Average time = 9.352 seconds (3 rounds), Encoding speed = 1156.12x

-j20: Average time = 8.942 seconds (3 rounds), Encoding speed = 1209.17x

-j22: Average time = 8.547 seconds (3 rounds), Encoding speed = 1264.96x

-j24: Average time = 8.219 seconds (3 rounds), Encoding speed = 1315.49x

-j26: Average time = 7.949 seconds (3 rounds), Encoding speed = 1360.17x

-j28: Average time = 7.850 seconds (3 rounds), Encoding speed = 1377.32x

-j30: Average time = 7.836 seconds (3 rounds), Encoding speed = 1379.73x

-j32: Average time = 7.791 seconds (3 rounds), Encoding speed = 1387.76x

-j34: Average time = 7.746 seconds (3 rounds), Encoding speed = 1395.82x

-j38: Average time = 7.819 seconds (3 rounds), Encoding speed = 1382.73x

-j42: Average time = 7.928 seconds (3 rounds), Encoding speed = 1363.72x

-j46: Average time = 7.963 seconds (3 rounds), Encoding speed = 1357.78x

Performance peak at -j32..-j34 here.