is using packs like over kill allowed?
Yup. They don't provide /that/ much of an advantage over normal weaponry, and certainly none that would impact a speedrun.
But some runs use the rpg, like the runs on diamond store, and those ones rely on the blast radius of the rpg, and I'm not even sure you could do the same strategy with non explosive weapons, because it would take time to shoot, and even half a second could affect the strategy working or not.
README: 3.9.6b and the Millisecond Timer
As of version 3.9.6b, Notoriety devs have added a function within the settings menu to turn off Safe Escape (removing the 3s timer) and enable the Speedrun Timer (which includes milliseconds). This is objectively the best way to measure runs, now, and importantly, isn't affected by Timer Glitch.
As
Recent runs
Level: Shadow Raid
Level: Shopping Spree
Level: Downtown Bank
Level: Transport
Level: The Ozela Heist