I have a puppy here who doesn't feel pain, it's the most unbelievable thing I have personally ever seen. I have talked to my trainer A LOT about it, I still find it confusing. Some dogs are just different than others, gimme a correction soft dog any day of the week, lol

To me there is a very big difference between injury when working and in drive and not reacting to it and a high pain tolerance and the dog that yelps or screams from a touch.
My Bullmastiff Basher was NOT a high drive dog at all but he had a very high pain tolerance. His sister was high drive out the roof and not reactive if she was hurt while in drive, but otherwise she had a normal tolerance level. Border Collies will work through anything but once the sheep are up will show a limp (or not) and end up with some injury. Likewise if I took a BC with a limp or in obvious pain and asked them to work they would without a backward glance.

As for yelping or screaming as if being murdered.....yeah, groom many maltese....scream like you are murdering them slowly. It is some dogs way of displaying disapproval. Same with some dogs that you correct or just lay a hand on a collar. At the boarding kennel you get to touch strange dogs. Some are fine, some are aggressive and some are screamers. "you can't make me 'cause you aren't my mother" attitudes. It is almost a personal space thing.