list specific chapter from a href tag?

[user]

From my tests, variable substitutions seem to be on exact par with command substitutions.

You're right..it doesn't matter much. And sometimes performance even gets worse with command substitution (because of the way it's compiled).

(except for those command substitutions which also involve variable substitutions at the same time, of course)

You mean when you take advantage of the fact that some commands both alter a variable and return the new value? ...That pretty much goes without saying (one less substitution to perform)