What we see is a close fit. The problem is the horse is standing still when you check the saddle for fit. As the horse moves his back is in a state of constant motion. The sweat pattern says a lot as does the temperature. When removing the saddle you have to be quick and run your hand front to back to feel for variations in heat. A warmer spot will have more heat. The middle area may be sweaty but cooler than front and back so we know there's a bit of bridging going on. Have I confused you further?
The people who put forth the "pad to fit" argument say it supports the horse's back in motion better. I like your advice of feeling for heat(not necessarily sweat), because it makes sense to me that if you were causing any stress or injury it would be warmer than surrounding tissues.
Unclearthur, I also agree that sweat can be a poor guide, because just variations in materials can alter sweat patterns, I have a saddle pad with thin gel inserts, my horse sweats far more under the gel patches than anywhere else.
How "perfect" does a fit have to be to get optimum performance out of your horse?
I recently read a book written by Grand Prix dressage rider that says she has 4 different custom made saddles for one horse. Each is "perfectly" fitted, but each is designed to spread her weight differently over the horse's back. She rotates the saddles because with the amount of riding she does, her horse is prone to getting sensitive spots on his back, she says by changing saddles daily, he doesn't get sore any more.
All I know is I tried a new saddle last week and my horse's feedback was instant! He was so much more willing to move, and carried his head at a much nicer level. Even though the saddle was heavier he seemed to have more energy throughout the ride.