From what I have heard from some folks that use dyno's quit a bit is 15 to 20 for a standard tranny and 20 to 25% for an auto.
High stall converters will give you some really bad dyno numbers. Most folks I know that have the lock-up torque converters always lock them up on the dyno and get much better numbers. And don't dyno in overdrive. The guys with overdrive put their cars in 3rd gear and have a switch to lock up the converter. I put out some bad numbers on the dyno with a 4500 stall, in fact I put out so bad of numbers I am upset.