The "widening kit" affects the bumpsteer.
Spacers don't affect the stock geometry or the stock handling. The DO make it harder to turn because you are trying to force the tire to rotate much farther out from the ball joints. Just flipping the rims makes it much more difficult to turn but worth it for the extra width. Extending the flipped wheels makes it even harder.
Bumpsteer is when the wheels are not pointed the same direction while the suspension goes through its travel. There is a way to design suspension geometry to have zero bumpsteer. All of the suspension parts have to converge at given angles but you can imagine fitting all that around the reality of building quads is difficult. Thus, the stock geometry has some bump steer. A manageable amount...
The widening kits keep the a-arms at their geometry but move their angles outwards (widening them) while the endpoints of the tie rods are not changed. This induces some bumpsteer. Most people who are trail riding wouldn't notice the extra bumpsteer mainly because most trail riders aren't flying and most bumps, both tires take at the same time. The real danger is when you are riding very fast and one side suspension goes through its arc and the other side ( because of weight transfer) does not. Racers in particular are particularly sensitive to bumpsteer because in addition to having one side suspension work harder than the other, they're side-by-side with someone a lot too. The unintended effect is that the tire going through its travel tries to steer in a different direction than the other tire and the front traction gets all silly. The quad will try to "dart" on the track.
