Using the turbine you already have, performance an usually be improved by raising it higher. How far from the ground is it now? What obstacles are nearby?
Please believe me that VAWT's have NO special immunity from turbulence, despite what 99% of the world will tell you. Keeping it away from buildings, trees, and the ground will increase the energy you collect. Sometimes the height of the tower or building is limited, so this answer doesn't fit all cases.
Sorry there isn't enough information to check calculations - that would be difficult in any case. Usually it is hard to measure wind speed accurately without special equipment. I would avoid trusting calculations if your uncertainty of wind speed is +/- 1 m/sec.