I don't know if I would call these rules, but these are some general observations.

At what performance level (if any) do you start to expect a reaction even with a strong pattern?

I mainly consider are the age/sex of the horse. Fillies are more likely to react to big efforts. So are younger horses. I don't know what kind of 'rule' you can make out of this. If a 2yo ran a zero I would be more inclined to mark him down a peg than a 4yo who ran a zero regardless of the pattern. Older horses are more able to run and pair the big numbers. Pattern is a factor in the sense that if the big races are not huge leaps forward, the horse may be able to stay at that level longer, but I think you need to knock down the 2yos running 2's and 3's regardless of pattern based on how they've fared over the past few years. I would imagine we will see the 2yo at some point who can run the 2's and 3's and persevere. He'll be a damn short price (more on this below).

Are there some "constants" that even the fastest, fittest, and strongest horses cannot exceed (regardless of pattern) without inducing a reaction?

Very few horses pair negative numbers, and I've never seen one run more than three in a row. That looks like about the limit to me. A few have been able to stay in the 1-2-negative number range for quite awhile (Skip Away, Cigar), but they are obvious exceptions. Had a great chat with Chuckles about this.

Are those constants changing over time (I know that sounds like an oxymoron) as horses seem to be getting faster and faster at a younger age?

Over the past few years, I've noticed that more top notch horses can pair the big efforts and negative numbers. A few of them have bounced a lot less that I would expect after two negative numbers as well. Things are definitely changing.

These big figure horses are going to get bet down to 1-5 most of the time. It's not like their quality is a secret. When I see them, I just try to figure out if they can get beat, and I don't make as much of an effort to figure out how much faster they can go, or how long they can keep it up. They can make for nice betting opportunities and that's my focus. Someday a horse will come along to break all the 'rules', but until then, I stick with the percentages I've seen and try to beat them when the time is right. HP