Correct me if I'm wrong, but unlike the game, I think the paddles only force a downshift (within limits), not an upshift. For example, if you click the upshift paddle (or use the shifter knob) from 1st to 2nd at, say, 3000RPM with full throttle, the car still won't shift until it reaches nearly max RPMs. Since you can only prevent an upshift, the very best you can do is to avoid a late shift.. something the transmission will do on its own when left in auto. Additionally, for downshifting, it's almost always faster to simply mash the throttle, which will automatically downshift (assuming you're not already doing over 130, in which case you can't downshift anyway). The only benefit I've seen from manual shifting is to use engine braking, and that doesn't work too great on an automatic anyway thanks to the torque converter. It's useful for reducing the required braking on declines, but to get any appreciable engine braking, you typically have to use a gear lower than an equivalent 6-speed manual, which means you're using very high RPMs, and that's just more wear and tear on the engine (which is a lot more expensive to service/replace than some brake pads/rotors).
Basically, in my opinion, the paddle shifters on the IS are close to useless.