I did this testing in response to a post on GFAQS. A big thanks to IKY for being a target for me. I normally don't use aim assist, and during previous tests in game, it never did anything. In fact, my accuracy would drop because I expected it to follow a target, but it never did. I found out why. The rumor was wider deadzones increase the effectiveness of wiimote aim assist. Both the ADS snap and the lock on Which turns out to be false. If you don't want to read it all, skip to the bold "In Summary" section.
Ok. So aim assist results.
The snap is the same with large deadzones or zero deadzones.
Aim assist only works within the maximum damage range of the gun you are using.
Recoil still took me off target after the snap and lock on was achieved on a stationary target.
I didn't notice the snap at first with zero deadzones because I'm right on target even with aim assist off and turning it on didn't change anything. I had to deliberately aim off target to notice it.
The lock on lasts approximately 5-6 steps. Rough guess.
The lock on will only hold if the wiimote isn't moved. At all. If the wiimote senses any movement, then the lock on will break. I forgot to test whether sensitivity settings play a factor. I'm guessing they do. Keeping the mote still enough to achieve that was difficult and very unnatural for me as I normally follow targets on my own, and I have high sensitivity. Edit: Cursor speed and sensitivity play a role as the lock breaks as soon as movement is registered.
Agility can outrun the lock on. My target ran across my screen. I locked on and held the trigger. I only got one hitmarker before the lock ended. It followed right behind the player for several shots. I did not test without agility but I may try tonight if my helper is up for it.
The ADS snap from the edge of it's range appears slow to me. Annoyingly slow. I can get on target faster on my own and follow a target better on my own than the lock on can. IKY with Agility doesn't outrun me aiming on my own.
During simulated normal gameplay, with both me and my target running around, I was unable to effectively aim with the large deadzones (lack of practice). So I gave that up really quick. Back on zero deadzones. I tried with and without aim assist and just aimed naturally. I just pretended I didn't have aim assist on just as I would in game. The effect was completely unnoticeable as expected. Since I don't stop moving the wii remote on a moving target, the lock on is either never achieved, or it is broken at the same instance that it is achieved. Even if it is achieved just the act of pulling the trigger breaks the lock under normal circumstances and it will not follow the target for me. This may be due to sensitivity and pulling the trigger causes a slight movement (though I was able to hold the mote still enough to get the lock while doing stationary testing, but again, it took effort). Either way the result was the same as it is in matches. I see no benefit from it playing as I normally do.
In summary: The snap to target is the same regardless of deadzone. Deadzone doesn't play a factor in how well the lock on holds. It is easier however to sit still and get the wiimote into the snap window with larger deadzones. I can't do it consistently at all while moving with large deadzones. The lock on will only hold if you don't move the wiimote, but doesn't follow a target nearly as well as I can without it. I can also move on target faster than the snap can from the edge of its range, and I'm normally on target already when I ADS. Just the act of me pulling the trigger is enough to break the lock on, and the lock on doesn't counteract recoil at a distance. So any correcting of the recoil will also break the lock on, but if you don't correct for it then you could miss.
You can see in killcams that it isn't effective all the time. Most of the wiimote users I see are all over the place in the killcam. Whereas my killcams are right on target to the point that no one knows I'm using a wiimote. They don't mention it until they see the helo pilot killcam.
In my opinion aim assist is much better for campers who can keep their hands perfectly still. I don't find it to be OP at all, and as expected, you can achieve much better results just by aiming on your own if you put in the effort.