Toyota changed the intake runner design for the 22r the give more low end torque by using a "swirl" design. If you look down the intake runner in the head with the intake manifold off on a 22r, you can only see half the valve! It curves around the valve to spin the incoming air. The 20r intake runner has a straight shot..no curves. The combustion chamber is a little smaller on the 20r, giving more compression. The only downfall is the 20r has 1mm smaller valves, Not a big deal for the amount of gain. To take full advantage, you need to install a Weber, cam, and header.To take full advantage, you can install 22 valves and seats in the 20 head, but the 22 valves are shorter. The rocker "boxes" can be milled down to correct this.
Also worth noting...Because a 20r head will not fit the 85-up blocks, Alot of people have figured out how to "hog out"(port) the newer heads for the same results. The newer(85-up) engines have higher compression and better combustion characteristics with a flat top piston.