Try it for yourself Ken. You can feel it when you oversheet if you are in tune with your boat, but having some telltales on the leech is much easier. If you try it while two boat tuning or racing, it is pretty obvious which mode is fastest. Especially so with a modern square top main.

With the time Smyth has on the water, he can feel when the main is over/under-sheeted and dont need to look. We have 6 years on the same boat with the same crew, we still check the leech telltales if we are uncertain. As for "why are you looking back there when going up wind?", We'll look anywhere if it makes us go faster..
If your leech telltales never flow aft when going upwind, put on more downhaul, trim mainsheet and go faster.