I have 18" wheels I think. I have the base model. So going with 19" rims will improve handling?
A bit, not sure how noticeable it would be by itself.
The overall diameter of your 18" tires is about 27". If you went to 19" wheels, the new tires would have lower sidewalls to maintain the same 27" overall diameter. So you end up with more wheel; which is very stiff because it is metal; and less tire, which is relatively soft / spongy because it is rubber. So there is less movement in the tire and the whole thing gets a bit more responsive. The ride also gets a bit firmer, or "harsher" to some people.
You could achieve the same thing by using lower sidewall tires on the 18" wheels. But this would also reduce the overall diameter, which is sort of like going with slightly steeper rear gearing; and throws off the speedometer a little bit. Most hard core road track cars use 18" wheels with the lower profile tires, because the 18" wheels are lighter than the 19's.