This is indeed a BS problem, I'm amazed this whole issue is still up for debate. It really is a no brainer. ALL the major clutch manufacturers recommend clearance, Ford (they choose NOT to change the design in spite of problems with it) and ONE SPF dealer with obviously limited experience dont'. Go figure this is even a debate...
Now where were we...
If you can push the fork back, with your hand on the end of the fork, and bottom the slave piston that's how much "force" it takes. Not "that much" in other words. So if the return spring is unable to pull the fork back far enough the angle of the pull may be binding the rod preventing free movement or the spring is not connected with enough "leverage" to do the job. It does require a fairly healthy spring, but not a killer spring!
So, assuming the spring is correct tension and mounted properly then some other force is pushing the slave piston out of it's bore. I could hazard some far out guess' none of them reasonable or logical though. The master is somehow maintaing pressure pushing fluid into the slave at rest. The slave hydraulic line to the master is blocked or restricted in some way. Once the slave gets some pressure it does not properly release the pressure (piece of debris in the line, blockage of some kind). The internal spring in the slave is "bad to the bone".