very true, its why if weight is an issue you want the lightest wheels possible to account for the increase in tyre weight.
you can get some of the benefits in 18 by going for a wider 18" wheel, and choosing a tyre with good strong stiff side walls or reinforced tyres, most reinforceds have the letters XL next to them. so if you see 2 identical tyres but ones more expensive its often becasue its reinfoced, or a different compond designed for a specific manufacturer. if its manufacturer specific, it has extra letters (code for each manfucaturer) the only ones who tend to have these are audi, jaguar, merc, porsche and ferrari are the most common.
you can go lower profile on zed 18"s but you might suffer TCS issues, but turning it off fixes that
if i was purely interested about tracking the zed for maximum benefit. i'd go with the lightest 18" with a wide stance and add lower profile tyres. i would have shed the unsprung weight improved the grip and reduced the tyre roll and increased steering and car feedback.