It does not support BPM at all. Fastest speed is 1 if you don't need PAL compatibility, otherwise it is 2. It is not recommended anyway, because music data will be huge. Slower speed - smaller data.
And it's pretty vital to keep things as small as possible when doing actual game soundtracks. For this game I only had 9 KB to squeeze the soundtrack into...hence why no tracks go under (over?) speed 5.
