Here is an article I copied
The idea behind a higher stall converter is to allow the engine to rev more freely up to the point where the powerband begins, and therefore, enable it accelerate from a stop under more power.
In simple terms, for best performance, the stall speed should be raised at least to the point where the torque curve is heading for it's peak. As a rule of thumb, the stall speed should be set to match the rpm at which the engine is making at least 80% of it's peak torque for a street driven vehicle.
As you can imagine, a vehicle that can accelerate from a stop with 80% of its peak torque will easily outperform the same vehicle that can only launch at 50% of its available torque.