Assuming that people that practice Stoic or Buddhist techniques are successful in diminishing their capacity to suffer, does that mean they are worth less morally than before they started?

It means that inducing some typically-harmful action on a Stoic is less harmful than inducing it on a normal person. For example, suppose you have a Stoic who no longer feels negative reactions to insults. If you insult her, she doesn't mind at all. It would be morally better to insult this person than to insult a typical person.

Let me put it this way: all suffering of equal degree is equally important, and the importance of suffering is proportional to its degree.

A lot of conclusions follow from this principle, including:

• animal suffering is important;
• if you have to do something to one of two beings and it will cause greater suffering to being A, then, all else being equal, you should do it to being B.