You can force a downshift, or even a double down shift by using the "kickdown" button under the pedal. Putting the selector in sports mode (3 in your car) will tell the computer to raise shift points. It will shift faster in this mode than it will if you manually select gears using the "steptronic" manual feature.
This is from experience.
with enough time and effort, you can learn to play your automatic computer, and get the tranny to hold gears, downshift, and upshift when you want it to. But it's difficult to master, as you're not truly in control of the gearbox - the tranny is. Search out the points in your car where it feels like it wants to shift, and whatnot, and just practice.
Or get a manual. :P
1998 BMW e36 M3 Sedan
Crap, I think I may be a yuppie...
2003 MINI Cooper