I think this is more of a tech support issue. The countdown doesn't trigger in negative lead-in. I checked and you need around 800 ms for the double time countdown, and that does not include audio lead-in which the game ignores. It doesn't matter how long the count down takes, it wont recognize lead-in as sufficient space to play the countdown.
Normal countdown speed is about 3 seconds and that starts after any leadin. So in theory a song with 3 seconds of leadin would get a countdown, but it would start instantly and would probably not be that great. But if you had an extra couple seconds in addition to that leadin it would work. IMO I would support increasing max possible leadin to 5-8 seconds and introduce the guideline to keep the leadin under 5 unless there is storyboard.