Hey, I have decided to buy NV just so that I can see if it is true that FO3 is bologna and NV is champagne, foie gras, and marinated truffles. I played FO3 once through, just enough of vanilla to get the idea and soon modded. However, I resented the unmerciful AI aiming skills. Let me tell you, in a game modded with FWE I can testify on the deadly ability of raiders that wield their broken weapons with the precision of cyborg neurosurgeons. Here is an example: I have just sneak attacked a raider jerk and focused on the doorway for his girl buddy who has surely been alerted and instantaneously communicated my position to the other raiders. Keep in mind this is while I am adjusting to FWE's deadly combat.

I see her lousy .32 pistol sticking out of the doorway and I immediately enter bullet time and use the scope of my Steyr AUG in anticipation of what should be a turkey shoot. Suddenly this woman who is wearing what looks like a beekeeper's head protector pops out and flies to the other side of the hall firing twice to cripple my head before I can react, and as I try in vain to get her in my crosshairs she turns and zips back to her original cover firing once more to headshot me while easily dodging my panic fire. This happens within in a fraction of a second. Situations like this happened so often that I was forced to yield shootouts and instead fall back and lay mines or hope that Amy Wong can distract them.

My questions about FNV are these:

1. Is FNV enemy AI aiming precision in vanilla really good, just right, not good, terrible, etc.?
2. Are there mods to improve or handicap enemy precision?

I've used a lot of mods to change enemy damage/accuracy armour for NV and have ended up going back to vanilla. I found the game was made far too easy for the player with one shot kills to the npc most of the time when the damage/accuracy settings were changed, and it made armour meaningless. The default game is not that bad and the npc's seem to have more trouble getting headshots. Their damage threshold system for armour is very good and armour does protect you better even if it's just leather. The ai of the npc is much improved over f03 but there is still a tendency for them to stand there and just shoot at you without taking cover.
They also still do that strafing while firing at you with great accuracy which can be really annoying as they're pretty fast. They now have a response to kills/dead bodies too which is new to New Vegas and makes a change to see.
I know exactly what you are talking about with the raiders, It was bad, I got headshot by a raider when the only thing showing were my feet while I was behind an object. I don't seem to notice that type of thing happening in this game or the split second killshots.
I used to use the medium settings for damage/accuracy when I played fallout 3 with FWE. Using similar settings for NV just didn't feel right.

High level encounters are a bit more difficult, there are some npc's who are pretty legendary in their headshot abilities, they're a great challenge though. Unarmed/Melee also is worth considering as it's much improved.
A mod to keep an eye on is Project Nevada, they were the creators of FWE. They're doing and have done a lot of testing so far for NV. Their initial mod is simply great, it is so professional, but I've found it's a little hard on a low end pc.
Anyway that's my opinion, it's much improved over FO3.

I'd like to see the AI's aiming skill be proportional to its weapon skill(s), however I'm not sure that's doable given that the computer is constantly running targeting calculations and always knows where you are.

It always bugs me when a cracked-out Fiend has competition-level accuracy with a popgun while running sideways at 30MPH, however if they missed all the time things would be too easy. Not that Fiends and other such targets are much of a challenge anyway, but still...