I was watching Phnom Penh '86 IL and I remembered that you can make replay on the roof where the briefcase will appear. After all the gang members on the roof have been killed and you are on foot, you can activate the replay and mission ending cutscene is triggered immediately. Current runs use clothes replay to do the weapon glitch. Maybe doing the briefcase warp instead of weapon glitch is a bit faster. I'm not speedrunner, so I can't tell for sure.

Found two ways to warp/complete mission objective using replays:

1. Guardian Angels: make a replay where the Colonel's Ruger will appear at the multi-store car park. After starting mission at the marina bay, watch replay and you appear at the car park and the Lance cutscene starts. Possible to save ~10 sec (including setting up the replay, which can be made on the way back after Treacherous Swine).

2. Phnom Penh '86: make replay on the roof where the briefcase will appear. After all the gang members on the roof have been killed and you are on foot, you can activate the replay and mission ending cutscene is triggered immediately. Probably won't save time, because you have to go to the roof anyways before the mission to record the replay. On the plus side you won't take any damage.

Other thoughts: I think this idea can be used for other missions that include picking up briefcases etc. One idea is Juju Scramble, where you can set up replay for pickiing up one of the cases. Haven't tested this tho.