larger rims are usually heavier (larger unsprung weight leads to potentially bouncing tires). They usually have a larger overall diameter as well (tire and rim together). This effectively lengthens your gears (slower accel generally) and makes it harder for your brakes to be effective.
In terms of cornering, unless you shocks are shot they shouldn't be too much of a problem unless you're loosing contact with the road (see first line lol)
usually the lower profile tire will improve cornering to some degee
either way...played out 18's are FTL