Hammers are cased, but not very deep. I don't think the triggers are heat treated at all. The real reason for that heavy trigger pull is the mainsprings used in most cap & ball revolvers. They didn't call them "thumb-busters" for nothing. They can be eased a little, but go too far and you'll get hammer blowback.
Cutting back the full-cock notch is a mistake, because the trigger will catch in the half-cock notch, and likely bust both the notch and trigger. If you want to reduce the hammer/trigger engagement drill a small hole behind the full-cock notch and insert a pin to act as a stop.
