Torque converter slipage will depend on the amount of power input at the time its measured. For example, if your car reaches max RPM before the end of the 1/4, your slippage will obvioulsly degrease significantly over pulling through the gears at max torque. In this example, the converter may look more efficient then it actually is.
To get a true picture of converter efficientcy, you need to compare slippage in an A/B comparison on the same combo.
Allan G.