How long a delay...interesting question. Hard for me to measure (or even guess) how much delay is happening in the screens (or in the video-distribution systems), and obviously a lot of factors can affect situations where audio and video sources are far apart (could be a fair amount of delay in a very large venue, or in an outdoor situation). Just to throw out a semi-educated guess, I'd say probably an adjustment range of 500ms (half-second) should cover most situations. I hadn't really thought about the effects and scratching...but couldn't at least the effects be compensated for? Or are you not reading the waveform data from SSL, and looking at a real-time audio "feed" instead? In any case, yes, I agree that being able to compensate for a sync problem would take priority.
