response is a little late but...
the bpm does not actually change the slider length per musical beat. So what I mean is that, if you have SV 1.4 on 180BPM, and SV1.4 on 140BPM, the length of the slider for a 1/2 tick is actually the same for both song speeds.
in terms of actually setting a SV, usually 1.4 - 1.6 ish is most often used i think (for insane maps). That speed also generally seems to flow the best (using 1.0x SV). This question is pretty redundant in that you need to match the SV to the song, so during kiai times, SV is usually increased, and during quite sections SV is usually decreased. This is not always the case depending on what the mapper wants, but either way it's up to you to decide, so yeah. There isn't some universal get out of jail free card here.
for other difficulties, the SV is generally lower than that. Again, up to your own discretion.