It doesn't look significant because the game is old and runs already pretty fast on modern hardware. If it was more gpu intensive game, a 20% difference could make a difference between a tolerably fluid framerate and a jerky framerate.
20% is quite significant. If it was 50% better, then we'd already be saying that DirectX performance is cr*p compared to OpenGL. But 20% is definitely a decent boost, especially since you'd expect DirectX to fare better than OpenGL.
To be fair though, if you would take a game that seriously stresses the GPU you wouldn't see a 20% difference. You would probably see about the same constant difference of about 0.5 ms.
For 60 FPS that's an increase to 61 FPS.
I don't think it's fair to compare DirectX and optimized OpenGL. After they optimized OpenGL implementation on Windows they got: 315/303 = 1.04. That is 4%.