[Rev]

OK, here's my take on converters. As mentioned by PKRWUD, the converter functions like slipping the clutch in a manual setup.

When we go for higher power in a given engine, we almost always do it at a higher peak RPM. That means higher RPM torque peak (almost always). and a major loss in low end torque. This requires a higher stall converter (automatic) to get into the power curve of that engine. We need to compensate for that loss of low end torque. Lower gears help with the torque multiplication also, but it it is possible to have too low of gears and too much stall speed for a given engine/ car wt, etc.

The reason cars came with low stall speeds OEM is because they were biased towards low end torque and low RPM potential power. This made for good driveability at the expense of performance.
Performance engines usually require higher stall speeds with automatics.

Rev