From my understanding a good converter will slip around 4-6% once past its stall speed.
IMO, I believe that is the optimum amount and is a "guage" of the converters actual efficiency. You see alot of comments regarding "loose" converters and people advising the use of a loose converter in some instances. To me its alittle mis-leading, doesn't make too much sense and should not be confused with actual stall speed. Once the optimum stall speed is chosen for the engine, I would expect the converter to get the revs up into that hi/max torque area that the stall speed was chosen for and then DRIVE the vehicle with the minimum amount of slippage. If its slipping excessively past its stall speed, it is loosing efficiency.
Converters that are "loose" may have a place on a hot street car, that you don't want to over-do the stall speed (due to cruise revs) but it is essentially still slipping to some extent especially with alittle load on the vehicle....which generates heat.