Using a DX lens on m43 via the 0.71x Speed Booster shows about 7% more image circle diameter than using the non-boosted lens on a DX camera. In most cases this has little or no effect on image quality - you just get a little wider field than normal. Note that if you shoot video the top and bottom of the image are cropped, so this effect goes away.
Did you use a hood with your 30/1.4? The only time I've seen such pronounced vignetting with a DX lens on m43 is with ultra-wide lenses having fixed hoods. FWIW, I've tried the new version of the Sigma 30/1.4 on the Speed Booster, and it doesn't behave this way at all, even with the hood attached and stopped-down to minimum aperture.