Stall Converter
As Adam said, raising the stall speed will let the motor get into it's power band quicker. If the motor make all it's power from 3500-6000rpm, and the Torque Converter stalls at 800rpm, then the engine will seem to bog when you mash on the gas. But if you slap in a TC that stalls at 3500rpm, the motor will very quickly rev to that rpm and be right in it's peak power range. It's similar to rev'ing the engine and dumping the clutch on a manual transmission car.
And there is more to TC's than just stall speed. You would be miles ahead talking to a reputable TC manufacturer to select one (or better yet custom build one) that matches your exact engine/car combination and driving habits. NEVER listen to recomendations from your buddies when it comes to selecting a stall speed.