Replacing a single gem with a Str/rating hybrid for a +20 (or greater) Str bonus. No Str loss, 20+ rating gain. Free lunch.
Replacing two gems with Str/rating hybrids for +30 Str bonus. You lose 10 Str for 40 rating. Minor net gain.
Replacing a single gem with a Str/rating hybrid for a +30 Mastery Rating bonus. The loss of 20 Str for 50 Mastery (20 from gem, 30 from bonus) may be a net DPS gain. This will depend on your own stat weighting.
A +20 Mastery Rating bonus is not worth a single hybrid gem. The loss of 20 Str for 40 Mastery is a net DPS loss.