The only way to unlock that is to beat the times of Rockstar, as you said. Here are the times:
1. Deal Breaker - Time To Beat 7:03
2. Hangman's NOOSE - Time To Beat 2:32
3. Bomb Da Base II - Time To Beat 5:32
Here are videos for each of them. Not embedding them so it won't be a spoiler. Click on them to view them. Be careful though, they might spoil your game so you're doing it at your own risk!