I don't fully understand that 5v "composite" sync thing. Composite refers to the entire video signal and ALL of its components. When you reduce sync levels, you reduce "lock" or stabilization of video on a tv/monitor. It sounds plausible what you suggest but verification of the required levels may need the use of a waveform monitor and vectorscope to verify those levels are indeed accurate. You can do the math and theory to select component values, but tolerances may affect the overall outcome.