edit: and 4+ digit bpms can't simulate negative bpms because it won't skip over intervening notes, which is basically the only reason you'd want to be using a negative bpm
The latest version of sm4(alpha4) has fake arrow feature. No reason to use negative bpm anymore.
EDIT: In that case, what's the highest bpm that can be safely used with stepmania? 1 million bpm to skip a measure, for instance, takes a quarter of a millisecond. Will 10 million work safely? 100 million?
That would depend on how many bpm changes there are, wouldn't it? I hope instant teleportation feature of some sort will be added in the future release.