Insane Difficulty, focusses heavily on a new Money System, a Re-Modernized Gun-Play and overall the difficulty of GTA SA. Not only tweaking the gameplay, but bringing changes to the world, to all assets, shops and NPCs.
I will describe in depth almost everything but first things first:

I'll start by giving the due respect and many thanks to a bunch of people as the mod simply wouldn't be of the same quality without their works:

To:
DK22Pac, Junior_Djjr, ThirteenAG, DubHaMlg777, Razor1096, Alvian9xpro, Rodent, Erron_Black211, MKKJ, Fastman92, Link2012, Madleg, Cifan and Whitetigerswt.
- Custom Scripts

To:
DavSte, Beenox, Yaz222, Mussy44, Miguel16310, Vadim M and CJ2000.
- Weapon Models.

You will be able to find in-dept credits inside the mod folders.

So let's talk about the changes:


GAMEPLAY/GUNPLAY:
CJ is not as resistant as before, Health Values have been dropped to a point he can only survive a couple of shots without Armour. Health and Damage pools are shared with NPCs so every fight is fair on both ends. Keep in mind that as the story progresses, enemies will start wearing Armour or just be a little bit tougher in general. With the exception of important characters, having all double the health and good Armour. Before continuing i would like to mention that CJ will gain fat faster, muscles slowly, and lose both rapidly, so taking good care for him is essential to get through the end.

Weapons have gone a huge rework, it's an entirely new system much closer to what you would expect in modern VideoGames. From the over-the-shoulders camera, to weapons shooting fast and precise with a huge enphasis on recoil.
I have kept the weapon's levelling system but with a small twist, generally all weapons are held with two hands and aimed down sight but CJ will wield all pistols/machine pistols in one hand at Gangster Level and one in each at Hitman Level. Walking while aiming has been made faster to give you a chance to dodge incoming bullets but the bigger the weapons the slower CJ is going to be.

Every gun-fight is fast paced and hectic.

WORLD:
Peds and vehicles are all untouched although they take more damage and drop less money.
Every weapon pickup has been deleted, both in mission and free-roam, with the exception of some melee weapons, cameras and sprays, done to give space to the new Money System and side activities. Earning money will be difficult in this mod, fewer missions will pay you and more often than not it will not cover the expenses, taking your time to do side-activities will be very beneficial. I've tweaked the prices of shopping items but the main focus was on Ammunation; Weapons prices are expensive and will give a fair ammount of ammo, so think carefully about every purchase. Not only that but almost all special items and weapons have been made available to purchase, with some melees too and new weapons as well, inspired by beta/cut content and previous GTAs.

NPCs:
I've took my time to edit and balance the entirety of NPCs in all missions, save the ones CJ doesn't really interact with. To keep in mind is their ability to take accurate shots in short burst and will wield varied weapons. They are able to use all of the Vanilla arsenal, Shotguns standardized as well Snipers in some occasions.

VISUALS:
Not much has been changed of the visual aspects, leaving that to you.
The only thing i've taken my time to do was applying 1024x1024 new textures to all the weapons (Vanilla and New), photo-realistic textures taken online and in some occasions with my hand-work.
You can replace them if you wish to, you will be able to find all the files inside the mod folder, but i assure you they fit well into the cannon world and works just fine with graphical enhancments mods.

EXTRA ACTIVITIES:
Since earning money has become essential, i've added some extra activities by modifying some Custom Scripts.
You will be able to mug pedestrians at gun point on the streets, rob local stores as well owning a chop shop in San Fiero.
Vanilla Activities will simply give less money.
Down below you will find a list of commands.


INSTALLATION:

This mod was put together inside a simple folder to be dropped inside the Modloader. So if you aren't using it, you will need to download it first.
And that's all about it, it will take care of itself and work smoothly without issues, although be careful about installing other custom scripted mods.
Remember to prioritize it in the Modloader's Mod Options once the game booted, above all mods, to prevent any issues.


LIST OF COMMANDS:

The new activites are simple to boot up:

To mug pedestrians simply aim your weapons at them, they will give far more money than dropped ones.
To rob stores you must wear your mask first, by pressing X. It has modified so that as long as you are wearing it, and not wanted, you can rob as many stores as you want.
The Chop Shop in San Fiero works simple as well, take a car and drive it inside the red circle, a cutscene will play before giving back control to you.
As a bonus, if you wear a hoodie you can put it over your head by pressing Z.