Bugs in these implementations have been fixed, code has been simplified and damage and falloff calculation has been updated to properly interpolate values. Features like aim delay and focus delay did not work at all or not as intended. The way the game determines how NPCs shoot their guns is ancient code that is not only needlessly complex but also suffers from oversights and broken features. What it does Improves how NPCs shoot their guns A lot of features that the vanilla game had at some point were broken or disabled with game updates and have been fixed or re-enabled. The changes this mod makes are not intended to make the game feel drastically different but to make it more consistent and replace some of the very outdated code with something that actually works and makes more sense. As such, the mod won't adds tons of new features and enemies and won't touch player balance or skills.
It is intended to build on what I think Overkill tried to accomplish rather than being a complete rebalance mod. Streamlined Heisting makes a lot of the game's enemy mechanics more consistent.