It's not quite that much.

One source: "The use of light-weight alloys in wheels reduces rotational mass. This means that less energy will be required to accelerate the wheel. Given that each pound of rotational mass lost provides an equivalent performance gain as a 10 pound reduction in vehicle weight, the benefits of light alloy wheels on vehicle performance cannot be overlooked.
For example:
A reduction in the weight of the rim/tire assembly of 5lbs x 4 (all around the car) is equivalent to a 200lb weight reduction in vehicle weight (thats worth 0.200 in the 1/4 mile)"

If you are really into increasing accleration, get 16's.

If you want the looks of 18s, just get them. Some lightweight 18's won't slow you down any more than those 17s. 26lbs for 17" is pretty heavy.

Also, as tire sidewall gets shorter, it generally gets thicker and heavier. even though an 18" tire has less sidewall, it's likely significantly heavier than a stock 15" tire (some due to wider width too).