In some cases, it certainly is worth the improvement. Performance improvements translate into $$$.
Knowing that "LC_ALL=C can improve performance but cause UTF problems" is awesome information to base a design on. A blanket statement like "never use LC_ALL=C" is not.
Short of a bug or other pathological issue, if you resort to messing with locale just to avoid a performance problem, you're likely to hide other issues that will cause problems whenever another user attempts to do the same thing and it breaks.
Again, not worth it -- if only for performance sake, generally speaking.
Knowing that "LC_ALL=C can improve performance but cause UTF problems" is awesome information to base a design on. A blanket statement like "never use LC_ALL=C" is not.